The sparse-checkout passes along argv and argc to its sub-command helper
functions. Many of these sub-commands do not yet take any command-line
options, and ignore those parameters.
Let's instead add empty option lists and make sure we call
parse_options(). That will give a useful error message for something
like:
git sparse-checkout list --nonsense
which currently just silently ignores the unknown option.
As a bonus, it also silences some -Wunused-parameter warnings.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Allow runtime upgrade of the repository format version, which needs
to be done carefully.
There is a rather unpleasant backward compatibility worry with the
last step of this series, but it is the right thing to do in the
longer term.
* xl/upgrade-repo-format:
check_repository_format_gently(): refuse extensions for old repositories
sparse-checkout: upgrade repository to version 1 when enabling extension
fetch: allow adding a filter after initial clone
repository: add a helper function to perform repository format upgrade
The 'extensions' configuration variable gets special meaning in the new
repository version, so when enabling the extension we should upgrade the
repository to version 1.
Signed-off-by: Xin Li <delphij@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
sparse-checkout's purpose is to update the working tree to have it
reflect a subset of the tracked files. As such, it shouldn't be
switching branches, making commits, downloading or uploading data, or
staging or unstaging changes. Other than updating the worktree, the
only thing sparse-checkout should touch is the SKIP_WORKTREE bit of the
index. In particular, this sets up a nice invariant: running
sparse-checkout will never change the status of any file in `git status`
(reflecting the fact that we only set the SKIP_WORKTREE bit if the file
is safe to delete, i.e. if the file is unmodified).
Traditionally, we did a _really_ bad job with this goal. The
predecessor to sparse-checkout involved manual editing of
.git/info/sparse-checkout and running `git read-tree -mu HEAD`. That
command would stage and unstage changes and overwrite dirty changes in
the working tree.
The initial implementation of the sparse-checkout command was no better;
it simply invoked `git read-tree -mu HEAD` as a subprocess and had the
same caveats, though this issue came up repeatedly in review comments
and workarounds for the problems were put in place before the feature
was merged[1, 2, 3, 4, 5, 6; especially see 4 & 6].
[1] https://lore.kernel.org/git/CABPp-BFT9A5n=_bx5LsjCvbogqwSjiwgr5amcjgbU1iAk4KLJg@mail.gmail.com/
[2] https://lore.kernel.org/git/CABPp-BEmwSwg4tgJg6nVG8a3Hpn_g-=ZjApZF4EiJO+qVgu4uw@mail.gmail.com/
[3] https://lore.kernel.org/git/CABPp-BFV7TA0qwZCQpHCqx9N+JifyRyuBQ-pZ_oGfe-NOgyh7A@mail.gmail.com/
[4] https://lore.kernel.org/git/CABPp-BHYCCD+Vx5fq35jH82eHc1-P53Lz_aGNpHJNcx9kg2K-A@mail.gmail.com/
[5] https://lore.kernel.org/git/CABPp-BF+JWYZfDqp2Tn4AEKVp4b0YMA=Mbz4Nz62D-gGgiduYQ@mail.gmail.com/
[6] https://lore.kernel.org/git/20191121163706.GV23183@szeder.dev/
However, these workarounds, in addition to disabling the feature in a
number of important cases, also missed one special case. I'll get back
to it later.
In the 2.27.0 cycle, the disabling of the feature was lifted by finally
replacing the internal equivalent of `git read-tree -mu HEAD` with
something that did what we wanted: the new update_sparsity() function in
unpack-trees.c that only ever updates SKIP_WORKTREE bits in the index
and updates the working tree to match. This new function handles all
the cases that were problematic for the old implementation, except that
it breaks the same special case that avoided the workarounds of the old
implementation, but broke it in a different way.
So...that brings us to the special case: a git clone performed with
--no-checkout. As per the meaning of the flag, --no-checkout does not
check out any branch, with the implication that you aren't on one and
need to switch to one after the clone. Implementationally, HEAD is
still set (so in some sense you are partially on a branch), but
* the index is "unborn" (non-existent)
* there are no files in the working tree (other than .git/)
* the next time git switch (or git checkout) is run it will run
unpack_trees with `initial_checkout` flag set to true.
It is not until you run, e.g. `git switch <somebranch>` that the index
will be written and files in the working tree populated.
With this special --no-checkout case, the traditional `read-tree -mu
HEAD` behavior would have done the equivalent of acting like checkout --
switch to the default branch (HEAD), write out an index that matches
HEAD, and update the working tree to match. This special case slipped
through the avoid-making-changes checks in the original sparse-checkout
command and thus continued there.
After update_sparsity() was introduced and used (see commit f56f31af03
("sparse-checkout: use new update_sparsity() function", 2020-03-27)),
the behavior for the --no-checkout case changed: Due to git's
auto-vivification of an empty in-memory index (see do_read_index() and
note that `must_exist` is false), and due to sparse-checkout's
update_working_directory() code to always write out the index after it
was done, we got a new bug. That made it so that sparse-checkout would
switch the repository from a clone with an "unborn" index (i.e. still
needing an initial_checkout), to one that had a recorded index with no
entries. Thus, instead of all the files appearing deleted in `git
status` being known to git as a special artifact of not yet being on a
branch, our recording of an empty index made it suddenly look to git as
though it was definitely on a branch with ALL files staged for deletion!
A subsequent checkout or switch then had to contend with the fact that
it wasn't on an initial_checkout but had a bunch of staged deletions.
Make sure that sparse-checkout changes nothing in the index other than
the SKIP_WORKTREE bit; in particular, when the index is unborn we do not
have any branch checked out so there is no sparsification or
de-sparsification work to do. Simply return from
update_working_directory() early.
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
If commands like merge or rebase materialize files as part of their work,
or a previous sparse-checkout command failed to update individual files
due to dirty changes, users may want a command to simply 'reapply' the
sparsity rules. Provide one.
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
setup_unpack_trees_porcelain() provides much improved error/warning
messages; instead of a message that assumes that there is only one path
with a given problem despite being used by code that intentionally is
grouping and showing errors together, it uses a message designed to be
used with groups of paths. For example, this transforms
error: Entry ' folder1/a
folder2/a
' not uptodate. Cannot update sparse checkout.
into
error: Cannot update sparse checkout: the following entries are not up to date:
folder1/a
folder2/a
In the past the suboptimal messages were never actually triggered
because we would error out if the working directory wasn't clean before
we even called unpack_trees(). The previous commit changed that,
though, so let's use the better error messages.
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Remove the equivalent of 'git read-tree -mu HEAD' in the sparse-checkout
codepaths for setting the SKIP_WORKTREE bits and instead use the new
update_sparsity() function.
Note that when an issue is hit, the error message splits 'error' and
'Cannot update sparse checkout' on separate lines. For now, we use two
greps to find both pieces of the error message but subsequent commits
will clean up the messages reported to the user.
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
commit e091228e17 ("sparse-checkout: update working directory
in-process", 2019-11-21) allowed passing a pre-defined set of patterns
to unpack_trees(). However, if o->pl was NULL, it would still read the
existing patterns and use those. If those patterns were read into a
data structure that was allocated, naturally they needed to be free'd.
However, despite the same function being responsible for knowing about
both the allocation and the free'ing, the logic for tracking whether to
free the pattern_list was hoisted to an outer function with an
additional flag in unpack_trees_options. Put the logic back in the
relevant function and discard the now unnecessary flag.
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
"git sparse-checkout" learned a new "add" subcommand.
* ds/sparse-add:
sparse-checkout: allow one-character directories in cone mode
sparse-checkout: work with Windows paths
sparse-checkout: create 'add' subcommand
sparse-checkout: extract pattern update from 'set' subcommand
sparse-checkout: extract add_patterns_from_input()
When using Windows, a user may run 'git sparse-checkout set A\B\C'
to add the Unix-style path A/B/C to their sparse-checkout patterns.
Normalizing the input path converts the backslashes to slashes before we
add the string 'A/B/C' to the recursive hashset.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
When using the sparse-checkout feature, a user may want to incrementally
grow their sparse-checkout pattern set. Allow adding patterns using a
new 'add' subcommand. This is not much different from the 'set'
subcommand, because we still want to allow the '--stdin' option and
interpret inputs as directories when in cone mode and patterns
otherwise.
When in cone mode, we are growing the cone. This may actually reduce the
set of patterns when adding directory A when A/B is already a directory
in the cone. Test the different cases: siblings, parents, ancestors.
When not in cone mode, we can only assume the patterns should be
appended to the sparse-checkout file.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
In anticipation of adding "add" and "remove" subcommands to the
sparse-checkout builtin, extract a modify_pattern_list() method from the
sparse_checkout_set() method. This command will read input from the
command-line or stdin to construct a set of patterns, then modify the
existing sparse-checkout patterns after a successful update of the
working directory.
Currently, the only way to modify the patterns is to replace all of the
patterns. This will be extended in a later update.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
In anticipation of extending the sparse-checkout builtin with "add"
and "remove" subcommands, extract the code that fills a pattern list
based on the input values. The input changes depending on the
presence of "--stdin" or the value of core.sparseCheckoutCone.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Add a function for inserting a C string into a strbuf. Use it
throughout the source to get rid of magic string length constants and
explicit strlen() calls.
Like strbuf_addstr(), implement it as an inline function to avoid the
implicit strlen() calls to cause runtime overhead.
Helped-by: Taylor Blau <me@ttaylorr.com>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The sparse-checkout patterns allow special globs according to
fnmatch(3). When writing cone-mode patterns for paths containing
these characters, they must be escaped.
Use is_glob_special() to check which characters must be escaped
this way, and add a path to the tests that contains all glob
characters at once. Note that ']' is not special, since the
initial bracket '[' is escaped.
Reported-by: Jeff King <peff@peff.net>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
When in cone mode, the 'git sparse-checkout list' subcommand lists
the directories included in the sparse cone. When these directories
contain odd characters, such as a backslash, then we need to use
C-style quotes similar to 'git ls-tree'.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
If a user somehow creates a directory with an asterisk (*) or backslash
(\), then the "git sparse-checkout set" command will struggle to provide
the correct pattern in the sparse-checkout file. When not in cone mode,
the provided pattern is written directly into the sparse-checkout file.
However, in cone mode we expect a list of paths to directories and then
we convert those into patterns.
Even more specifically, the goal is to always allow the following from
the root of a repo:
git ls-tree --name-only -d HEAD | git sparse-checkout set --stdin
The ls-tree command provides directory names with an unescaped asterisk.
It also quotes the directories that contain an escaped backslash. We
must remove these quotes, then keep the escaped backslashes.
Use unquote_c_style() when parsing lines from stdin. Command-line
arguments will be parsed as-is, assuming the user can do the correct
level of escaping from their environment to match the exact directory
names.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
If a user somehow creates a directory with an asterisk (*) or backslash
(\), then the "git sparse-checkout set" command will struggle to provide
the correct pattern in the sparse-checkout file. When not in cone mode,
the provided pattern is written directly into the sparse-checkout file.
However, in cone mode we expect a list of paths to directories and then
we convert those into patterns.
However, there is some care needed for the timing of these escapes. The
in-memory pattern list is used to update the working directory before
writing the patterns to disk. Thus, we need the command to have the
unescaped names in the hashsets for the cone comparisons, then escape
the patterns later.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The 'git init' command creates the ".git/info" directory and fills it
with some default files. However, 'git worktree add' does not create
the info directory for that worktree. This causes a problem when running
"git sparse-checkout init" inside a worktree. While care was taken to
allow the sparse-checkout config to be specific to a worktree, this
initialization was untested.
Safely create the leading directories for the sparse-checkout file. This
is the safest thing to do even without worktrees, as a user could delete
their ".git/info" directory and expect Git to recover safely.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
When core.sparseCheckoutCone is enabled, the 'git sparse-checkout set'
command takes a list of directories as input, then creates an ordered
list of sparse-checkout patterns such that those directories are
recursively included and all sibling entries along the parent directories
are also included. Listing the patterns is less user-friendly than the
directories themselves.
In cone mode, and as long as the patterns match the expected cone-mode
pattern types, change the output of 'git sparse-checkout list' to only
show the directories that created the patterns.
With this change, the following piped commands would not change the
working directory:
git sparse-checkout list | git sparse-checkout set --stdin
The only time this would not work is if core.sparseCheckoutCone is
true, but the sparse-checkout file contains patterns that do not
match the expected pattern types for cone mode.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
When a user uses the sparse-checkout feature in cone mode, they
add patterns using "git sparse-checkout set <dir1> <dir2> ..."
or by using "--stdin" to provide the directories line-by-line over
stdin. This behaviour naturally looks a lot like the way a user
would type "git add <dir1> <dir2> ..."
If core.ignoreCase is enabled, then "git add" will match the input
using a case-insensitive match. Do the same for the sparse-checkout
feature.
Perform case-insensitive checks while updating the skip-worktree
bits during unpack_trees(). This is done by changing the hash
algorithm and hashmap comparison methods to optionally use case-
insensitive methods.
When this is enabled, there is a small performance cost in the
hashing algorithm. To tease out the worst possible case, the
following was run on a repo with a deep directory structure:
git ls-tree -d -r --name-only HEAD |
git sparse-checkout set --stdin
The 'set' command was timed with core.ignoreCase disabled or
enabled. For the repo with a deep history, the numbers were
core.ignoreCase=false: 62s
core.ignoreCase=true: 74s (+19.3%)
For reproducibility, the equivalent test on the Linux kernel
repository had these numbers:
core.ignoreCase=false: 3.1s
core.ignoreCase=true: 3.6s (+16%)
Now, this is not an entirely fair comparison, as most users
will define their sparse cone using more shallow directories,
and the performance improvement from eb42feca97 ("unpack-trees:
hash less in cone mode" 2019-11-21) can remove most of the
hash cost. For a more realistic test, drop the "-r" from the
ls-tree command to store only the first-level directories.
In that case, the Linux kernel repository takes 0.2-0.25s in
each case, and the deep repository takes one second, plus or
minus 0.05s, in each case.
Thus, we _can_ demonstrate a cost to this change, but it is
unlikely to matter to any reasonable sparse-checkout cone.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The index-merge performed by 'git sparse-checkout' will erase any staged
changes, which can lead to data loss. Prevent these attempts by requiring
a clean 'git status' output.
Helped-by: Szeder Gábor <szeder.dev@gmail.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The 'git sparse-checkout init' subcommand previously wrote directly
to the sparse-checkout file and then updated the working directory.
This may fail if there are modified files not included in the initial
pattern set. However, that left a populated sparse-checkout file.
Use the in-process working directory update to guarantee that the
init subcommand only changes the sparse-checkout file if the working
directory update succeeds.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
If two 'git sparse-checkout set' subcommands are launched at the
same time, the behavior can be unexpected as they compete to write
the sparse-checkout file and update the working directory.
Take a lockfile around the writes to the sparse-checkout file. In
addition, acquire this lock around the working directory update
to avoid two commands updating the working directory in different
ways.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The 'git sparse-checkout disable' subcommand returns a user to a
full working directory. The old process for doing this required
updating the sparse-checkout file with the "/*" pattern and then
updating the working directory with core.sparseCheckout enabled.
Finally, the sparse-checkout file could be removed and the config
setting disabled.
However, it is valuable to keep a user's sparse-checkout file
intact so they can re-enable the sparse-checkout they previously
used with 'git sparse-checkout init'. This is now possible with
the in-process mechanism for updating the working directory.
Reported-by: Szeder Gábor <szeder.dev@gmail.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The sparse-checkout builtin used 'git read-tree -mu HEAD' to update the
skip-worktree bits in the index and to update the working directory.
This extra process is overly complex, and prone to failure. It also
requires that we write our changes to the sparse-checkout file before
trying to update the index.
Remove this extra process call by creating a direct call to
unpack_trees() in the same way 'git read-tree -mu HEAD' does. In
addition, provide an in-memory list of patterns so we can avoid
reading from the sparse-checkout file. This allows us to test a
proposed change to the file before writing to it.
An earlier version of this patch included a bug when the 'set' command
failed due to the "Sparse checkout leaves no entry on working directory"
error. It would not rollback the index.lock file, so the replay of the
old sparse-checkout specification would fail. A test in t1091 now
covers that scenario.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
If a user provides folders A/ and A/B/ for inclusion in a cone-mode
sparse-checkout file, the parsing logic will notice that A/ appears
both as a "parent" type pattern and as a "recursive" type pattern.
This is unexpected and hence will complain via a warning and revert
to the old logic for checking sparse-checkout patterns.
Prevent this from happening accidentally by sanitizing the folders
for this type of inclusion in the 'git sparse-checkout' builtin.
This happens in two ways:
1. Do not include any parent patterns that also appear as recursive
patterns.
2. Do not include any recursive patterns deeper than other recursive
patterns.
In order to minimize duplicate code for scanning parents, create
hashmap_contains_parent() method. It takes a strbuf buffer to
avoid reallocating a buffer when calling in a tight loop.
Helped-by: Eric Wong <e@80x24.org>
Helped-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
To make the cone pattern set easy to use, update the behavior of
'git sparse-checkout (init|set)'.
Add '--cone' flag to 'git sparse-checkout init' to set the config
option 'core.sparseCheckoutCone=true'.
When running 'git sparse-checkout set' in cone mode, a user only
needs to supply a list of recursive folder matches. Git will
automatically add the necessary parent matches for the leading
directories.
When testing 'git sparse-checkout set' in cone mode, check the
error stream to ensure we do not see any errors. Specifically,
we want to avoid the warning that the patterns do not match
the cone-mode patterns.
Helped-by: Eric Wong <e@80x24.org>
Helped-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The instructions for disabling a sparse-checkout to a full
working directory are complicated and non-intuitive. Add a
subcommand, 'git sparse-checkout disable', to perform those
steps for the user.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The 'git sparse-checkout set' subcommand takes a list of patterns
and places them in the sparse-checkout file. Then, it updates the
working directory to match those patterns. For a large list of
patterns, the command-line call can get very cumbersome.
Add a '--stdin' option to instead read patterns over standard in.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The 'git sparse-checkout set' subcommand takes a list of patterns
as arguments and writes them to the sparse-checkout file. Then, it
updates the working directory using 'git read-tree -mu HEAD'.
The 'set' subcommand will replace the entire contents of the
sparse-checkout file. The write_patterns_and_update() method is
extracted from cmd_sparse_checkout() to make it easier to implement
'add' and/or 'remove' subcommands in the future.
If the core.sparseCheckout config setting is disabled, then enable
the config setting in the worktree config. If we set the config
this way and the sparse-checkout fails, then re-disable the config
setting.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
When someone wants to clone a large repository, but plans to work
using a sparse-checkout file, they either need to do a full
checkout first and then reduce the patterns they included, or
clone with --no-checkout, set up their patterns, and then run
a checkout manually. This requires knowing a lot about the repo
shape and how sparse-checkout works.
Add a new '--sparse' option to 'git clone' that initializes the
sparse-checkout file to include the following patterns:
/*
!/*/
These patterns include every file in the root directory, but
no directories. This allows a repo to include files like a
README or a bootstrapping script to grow enlistments from that
point.
During the 'git sparse-checkout init' call, we must first look
to see if HEAD is valid, since 'git clone' does not have a valid
HEAD at the point where it initializes the sparse-checkout. The
following checkout within the clone command will create the HEAD
ref and update the working directory correctly.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Getting started with a sparse-checkout file can be daunting. Help
users start their sparse enlistment using 'git sparse-checkout init'.
This will set 'core.sparseCheckout=true' in their config, write
an initial set of patterns to the sparse-checkout file, and update
their working directory.
Make sure to use the `extensions.worktreeConfig` setting and write
the sparse checkout config to the worktree-specific config file.
This avoids confusing interactions with other worktrees.
The use of running another process for 'git read-tree' is sub-
optimal. This will be removed in a later change.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The sparse-checkout feature is mostly hidden to users, as its
only documentation is supplementary information in the docs for
'git read-tree'. In addition, users need to know how to edit the
.git/info/sparse-checkout file with the right patterns, then run
the appropriate 'git read-tree -mu HEAD' command. Keeping the
working directory in sync with the sparse-checkout file requires
care.
Begin an effort to make the sparse-checkout feature a porcelain
feature by creating a new 'git sparse-checkout' builtin. This
builtin will be the preferred mechanism for manipulating the
sparse-checkout file and syncing the working directory.
The documentation provided is adapted from the "git read-tree"
documentation with a few edits for clarity in the new context.
Extra sections are added to hint toward a future change to
a more restricted pattern set.
Helped-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
