"git repack" has been taught to generate multi-pack reachability
bitmaps.
* tb/repack-write-midx:
test-read-midx: fix leak of bitmap_index struct
builtin/repack.c: pass `--refs-snapshot` when writing bitmaps
builtin/repack.c: make largest pack preferred
builtin/repack.c: support writing a MIDX while repacking
builtin/repack.c: extract showing progress to a variable
builtin/repack.c: rename variables that deal with non-kept packs
builtin/repack.c: keep track of existing packs unconditionally
midx: preliminary support for `--refs-snapshot`
builtin/multi-pack-index.c: support `--stdin-packs` mode
midx: expose `write_midx_file_only()` publicly
The "--preserve-merges" option of "git rebase" has been removed.
* js/retire-preserve-merges:
sequencer: restrict scope of a formerly public function
rebase: remove a no-longer-used function
rebase: stop mentioning the -p option in comments
rebase: remove obsolete code comment
rebase: drop the internal `rebase--interactive` command
git-svn: drop support for `--preserve-merges`
rebase: drop support for `--preserve-merges`
pull: remove support for `--rebase=preserve`
tests: stop testing `git rebase --preserve-merges`
remote: warn about unhandled branch.<name>.rebase values
t5520: do not use `pull.rebase=preserve`
The mergesort implementation used to sort linked list has been
optimized.
* rs/mergesort:
test-mergesort: use repeatable random numbers
mergesort: use ranks stack
p0071: test performance of llist_mergesort()
p0071: measure sorting of already sorted and reversed files
test-mergesort: add unriffle_skewed mode
test-mergesort: add unriffle mode
test-mergesort: add generate subcommand
test-mergesort: add test subcommand
test-mergesort: add sort subcommand
test-mergesort: use strbuf_getline()
When "git cmd -h" shows more than one line of usage text (e.g.
the cmd subcommand may take sub-sub-command), parse-options API
learned to align these lines, even across i18n/l10n.
* ab/align-parse-options-help:
parse-options: properly align continued usage output
git rev-parse --parseopt tests: add more usagestr tests
send-pack: properly use parse_options() API for usage string
parse-options API users: align usage output in C-strings
Teach "git help -c" into helping the command line completion of
configuration variables.
* ab/help-config-vars:
help: move column config discovery to help.c library
help / completion: make "git help" do the hard work
help tests: test --config-for-completion option & output
help: simplify by moving to OPT_CMDMODE()
help: correct logic error in combining --all and --guides
help: correct logic error in combining --all and --config
help tests: add test for --config output
help: correct usage & behavior of "git help --guides"
help: correct the usage string in -h and documentation
Built-in fsmonitor (part 1).
* jh/builtin-fsmonitor-part1:
t/helper/simple-ipc: convert test-simple-ipc to use start_bg_command
run-command: create start_bg_command
simple-ipc/ipc-win32: add Windows ACL to named pipe
simple-ipc/ipc-win32: add trace2 debugging
simple-ipc: move definition of ipc_active_state outside of ifdef
simple-ipc: preparations for supporting binary messages.
trace2: add trace2_child_ready() to report on background children
Updates to the tests in t0000 to test the test framework.
* ab/lib-subtest:
test-lib tests: get rid of copy/pasted mock test code
test-lib tests: assert 1 exit code, not non-zero
test-lib tests: refactor common part of check_sub_test_lib_test*()
test-lib tests: avoid subshell for "test_cmp" for readability
test-lib tests: don't provide a description for the sub-tests
test-lib tests: split up "write and run" into two functions
test-lib tests: move "run_sub_test" to a new lib-subtest.sh
Various fixes in code paths that move untracked files away to make room.
* en/removing-untracked-fixes:
Documentation: call out commands that nuke untracked files/directories
Comment important codepaths regarding nuking untracked files/dirs
unpack-trees: avoid nuking untracked dir in way of locally deleted file
unpack-trees: avoid nuking untracked dir in way of unmerged file
Change unpack_trees' 'reset' flag into an enum
Remove ignored files by default when they are in the way
unpack-trees: make dir an internal-only struct
unpack-trees: introduce preserve_ignored to unpack_trees_options
read-tree, merge-recursive: overwrite ignored files by default
checkout, read-tree: fix leak of unpack_trees_options.dir
t2500: add various tests for nuking untracked files
"git grep --recurse-submodules" takes trees and blobs from the
submodule repository, but the textconv settings when processing a
blob from the submodule is not taken from the submodule repository.
A test is added to demonstrate the issue, without fixing it.
* mt/grep-submodule-textconv:
grep: demonstrate bug with textconv attributes and submodules
"git add", "git mv", and "git rm" have been adjusted to avoid
updating paths outside of the sparse-checkout definition unless
the user specifies a "--sparse" option.
* ds/add-rm-with-sparse-index:
advice: update message to suggest '--sparse'
mv: refuse to move sparse paths
rm: skip sparse paths with missing SKIP_WORKTREE
rm: add --sparse option
add: update --renormalize to skip sparse paths
add: update --chmod to skip sparse paths
add: implement the --sparse option
add: skip tracked paths outside sparse-checkout cone
add: fail when adding an untracked sparse file
dir: fix pattern matching on dirs
dir: select directories correctly
t1092: behavior for adding sparse files
t3705: test that 'sparse_entry' is unstaged
Code clean-up in "git difftool".
* da/difftool:
difftool: add a missing space to the run_dir_diff() comments
difftool: remove an unnecessary call to strbuf_release()
difftool: refactor dir-diff to write files using helper functions
difftool: create a tmpdir path without repeated slashes
CI learns to run the leak sanitizer builds.
* ab/sanitize-leak-ci:
tests: add a test mode for SANITIZE=leak, run it in CI
Makefile: add SANITIZE=leak flag to GIT-BUILD-OPTIONS
The ref iteration code used to optionally allow dangling refs to be
shown, which has been tightened up.
* jk/ref-paranoia:
refs: drop "broken" flag from for_each_fullref_in()
ref-filter: drop broken-ref code entirely
ref-filter: stop setting FILTER_REFS_INCLUDE_BROKEN
repack, prune: drop GIT_REF_PARANOIA settings
refs: turn on GIT_REF_PARANOIA by default
refs: omit dangling symrefs when using GIT_REF_PARANOIA
refs: add DO_FOR_EACH_OMIT_DANGLING_SYMREFS flag
refs-internal.h: reorganize DO_FOR_EACH_* flag documentation
refs-internal.h: move DO_FOR_EACH_* flags next to each other
t5312: be more assertive about command failure
t5312: test non-destructive repack
t5312: create bogus ref as necessary
t5312: drop "verbose" helper
t5600: provide detached HEAD for corruption failures
t5516: don't use HEAD ref for invalid ref-deletion tests
t7900: clean up some more broken refs
Test updates.
* sg/test-split-index-fix:
read-cache: fix GIT_TEST_SPLIT_INDEX
tests: disable GIT_TEST_SPLIT_INDEX for sparse index tests
read-cache: look for shared index files next to the index, too
t1600-index: disable GIT_TEST_SPLIT_INDEX
t1600-index: don't run git commands upstream of a pipe
t1600-index: remove unnecessary redirection
"git multi-pack-index write --bitmap" learns to propagate the
hashcache from original bitmap to resulting bitmap.
* tb/midx-write-propagate-namehash:
t5326: test propagating hashcache values
p5326: generate pack bitmaps before writing the MIDX bitmap
p5326: don't set core.multiPackIndex unnecessarily
p5326: create missing 'perf-tag' tag
midx.c: respect 'pack.writeBitmapHashcache' when writing bitmaps
pack-bitmap.c: propagate namehash values from existing bitmaps
t/helper/test-bitmap.c: add 'dump-hashes' mode
Use MINSTD to generate pseudo-random numbers consistently instead of
using rand(3), whose output can vary from system to system, and reset
its seed before filling in the test values. This gives repeatable
results across versions and systems, which simplifies sharing and
comparing of results between developers.
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
In read_midx_preferred_pack(), we open the bitmap index but never free
it. This isn't a big deal since this is just a test helper, and we exit
immediately after, but since we're trying to keep our leak-checking tidy
now, it's worth fixing.
Signed-off-by: Jeff King <peff@peff.net>
Acked-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Code cleanup.
* ab/repo-settings-cleanup:
repository.h: don't use a mix of int and bitfields
repo-settings.c: simplify the setup
read-cache & fetch-negotiator: check "enum" values in switch()
environment.c: remove test-specific "ignore_untracked..." variable
wrapper.c: add x{un,}setenv(), and use xsetenv() in environment.c
Regression in "git commit-graph" command line parsing has been
corrected.
* tb/commit-graph-usage-fix:
builtin/multi-pack-index.c: disable top-level --[no-]progress
builtin/commit-graph.c: don't accept common --[no-]progress
"git rebase <upstream> <tag>" failed when aborted in the middle, as
it mistakenly tried to write the tag object instead of peeling it
to HEAD.
* pw/rebase-of-a-tag-fix:
rebase: dereference tags
rebase: use lookup_commit_reference_by_name()
rebase: use our standard error return value
t3407: rework rebase --quit tests
t3407: strengthen rebase --abort tests
t3407: use test_path_is_missing
t3407: rename a variable
t3407: use test_cmp_rev
t3407: use test_commit
t3407: run tests in $TEST_DIRECTORY
More code paths that use the hack to add submodule's object
database to the set of alternate object store have been cleaned up.
* jt/add-submodule-odb-clean-up:
revision: remove "submodule" from opt struct
repository: support unabsorbed in repo_submodule_init
submodule: remove unnecessary unabsorbed fallback
When using `test_size` with `wc -c`, users on certain platforms can run
into issues when `wc` emits leading space characters in its output,
which confuses get_times.
Callers could switch to use test_file_size instead of `wc -c` (the
former never prints leading space characters, so will always work with
test_size regardless of platform), but this is an easy enough spot to
miss that we should teach get_times to be more tolerant of the input it
accepts.
Teach get_times to do just that by stripping any leading space
characters.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
b3dfeebb92 (rebase: avoid computing unnecessary patch IDs, 2016-07-29)
added a perf test that calls tac(1) from GNU core utilities. Support
systems without it by reversing the generated list using sort -nr
instead. sort(1) with options -n and -r is already used in other tests.
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Protocol v0 clients can get stuck parsing a malformed feature line.
* ah/connect-parse-feature-v0-fix:
connect: also update offset for features without values
Sensitive data in the HTTP trace were supposed to be redacted, but
we failed to do so in HTTP/2 requests.
* jk/http-redact-fix:
http: match headers case-insensitively when redacting
"git cvsserver" had a long-standing bug in its authentication code,
which has finally been corrected (it is unclear and is a separate
question if anybody is seriously using it, though).
* cb/cvsserver:
Documentation: cleanup git-cvsserver
git-cvsserver: protect against NULL in crypt(3)
git-cvsserver: use crypt correctly to compare password hashes
"git clone" from a repository whose HEAD is unborn into a bare
repository didn't follow the branch name the other side used, which
is corrected.
* jk/clone-unborn-head-in-bare:
clone: handle unborn branch in bare repos
"git stash", where the tentative change involves changing a
directory to a file (or vice versa), was confused, which has been
corrected.
* en/stash-df-fix:
stash: restore untracked files AFTER restoring tracked files
stash: avoid feeding directories to update-index
t3903: document a pair of directory/file bugs
To prevent the race described in an earlier patch, generate and pass a
reference snapshot to the multi-pack bitmap code, if we are writing one
from `git repack`.
This patch is mostly limited to creating a temporary file, and then
calling for_each_ref(). Except we try to minimize duplicates, since
doing so can drastically reduce the size in network-of-forks style
repositories. In the kernel's fork network (the repository containing
all objects from the kernel and all its forks), deduplicating the
references drops the snapshot size from 934 MB to just 12 MB.
But since we're handling duplicates in this way, we have to make sure
that we preferred references (those listed in pack.preferBitmapTips)
before non-preferred ones (to avoid recording an object which is pointed
at by a preferred tip as non-preferred).
We accomplish this by doing separate passes over the references: first
visiting each prefix in pack.preferBitmapTips, and then over the rest of
the references.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Check if sorting takes advantage of already sorted or reversed content,
or if that corner case actually decreases performance, like it would for
a simplistic quicksort implementation.
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Add a mode that turns a sorted list into adversarial input for a
bottom-up mergesort implementation that doubles the length of sorted
sublists at each level -- like our llist_mergesort().
While unriffle mode splits the list in half at each recursion step,
unriffle_skewed splits it into 2^l items and the rest, with 2^l being
the highest power of two smaller than the number of items and thus
2^l >= rest. The rest is unriffled with the tail of the first half to
require a merge to compare the maximum number of elements.
It complements the unriffle mode, which targets balanced merges. If
the number of elements is a power of two then both actually produce the
same result, as 2^l == rest == n/2 at each recursion step in that case.
Here are the results:
$ t/helper/test-tool mergesort test | awk '
$7 > max[$3] {max[$3] = $7; line[$3] = $0}
END {for (n in line) print line[n]}
'
distribut mode n m get_next set_next compare verdict
sawtooth unriffle_skewed 100 128 1184 700 589 OK
sawtooth unriffle_skewed 1023 1024 16373 10230 9207 OK
sawtooth unriffle 1024 1024 16384 10240 9217 OK
sawtooth unriffle_skewed 1025 2048 18454 11275 10241 OK
The sawtooth distribution with m>=n produces a sorted list and
unriffle_skewed mode turns it into adversarial input for unbalanced
merges, which it wins in all cases except for n=1024 -- the resulting
list is the same, but unriffle is tested before unriffle_skewed, so its
result is selected by the AWK script.
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Add a mode that turns sorted items into adversarial input for mergesort.
Do that by running mergesort in reverse and rearranging the items in
such a way that each merge needs the maximum number of operations to
undo it.
To riffle is a card shuffling technique and involves splitting a deck
into two and then to interleave them. A perfect riffle takes one card
from each half in turn. That's similar to the most expensive merge,
which has to take one item from each sublist in turn, which requires the
maximum number of comparisons (n-1).
So unriffle does that in reverse, i.e. it generates the first sublist
out of the items at even indexes and the second sublist out of the items
at odd indexes, without changing their order in any other way. Done
recursively until we reach the trivial sublist length of one, this
twists the list into an order that requires the maximum effort for
mergesort to untangle.
As a baseline, here are the rand distributions with the highest number
of comparisons from "test-tool mergesort test":
$ t/helper/test-tool mergesort test | awk '
NR > 1 && $1 != "rand" {next}
$7 > max[$3] {max[$3] = $7; line[$3] = $0}
END {for (n in line) print line[n]}
'
distribut mode n m get_next set_next compare verdict
rand copy 100 32 1184 700 569 OK
rand reverse_1st_half 1023 256 16373 10230 8976 OK
rand reverse_1st_half 1024 512 16384 10240 8993 OK
rand dither 1025 64 18454 11275 9970 OK
And here are the most expensive ones overall:
$ t/helper/test-tool mergesort test | awk '
$7 > max[$3] {max[$3] = $7; line[$3] = $0}
END {for (n in line) print line[n]}
'
distribut mode n m get_next set_next compare verdict
stagger reverse 100 64 1184 700 580 OK
sawtooth unriffle 1023 1024 16373 10230 9179 OK
sawtooth unriffle 1024 1024 16384 10240 9217 OK
stagger unriffle 1025 2048 18454 11275 10241 OK
The sawtooth distribution with m>=n generates a sorted list. The
unriffle mode is designed to turn that into adversarial input for
mergesort, and that checks out for n=1023 and n=1024, where it produces
the list that requires the most comparisons.
Item counts that are not powers of two have other winners, and that's
because unriffle recursively splits lists into equal-sized halves, while
llist_mergesort() splits them into the biggest power of two smaller than
n and the rest, e.g. for n=1025 it sorts the first 1024 separately and
finally merges them to the last item.
So unriffle mode works as designed for the intended use case, but to
consistently generate adversarial input for unbalanced merges we need
something else.
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Add a subcommand for printing test data. It can be used to generate
special test cases and feed them into the sort subcommand or sort(1) for
performance measurements. It may also be useful to illustrate the
effect of distributions, modes and their parameters.
It generates n integers with the specified distribution and its
distribution-specific parameter m. E.g. m is the maximum value for
the plateau distribution and the length and height of individual teeth
of the sawtooth distribution.
The generated values are printed as zero-padded eight-digit hexadecimal
numbers to make sure alphabetic and numeric order are the same.
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Adapt the qsort certification program from "Engineering a Sort Function"
by Bentley and McIlroy for testing our linked list sort function. It
generates several lists with various distribution patterns and counts
the number of operations llist_mergesort() needs to order them. It
compares the result to the output of a trusted sort function (qsort(1))
and also checks if the sort is stable.
Also add a test script that makes use of the new subcommand.
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Give the code for sorting a text file its own sub-command. This allows
extending the helper, which we'll do in the following patches.
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Strip line ending characters to make sure empty lines are sorted like
sort(1) does.
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The paths generated by difftool are passed to user-facing diff tools.
Using paths with repeated slashes in them is a cosmetic blemish that
is exposed to users and can be avoided.
Use a strbuf to create the buffer used for the dir-diff tmpdir.
Strip trailing slashes from the value read from TMPDIR to avoid
repeated slashes in the generated paths.
Adjust the error handling to avoid leaking strbufs and to avoid
returning -1 to cmd_main().
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
In some circumstances, "git grep --textconv --recurse-submodules"
ignores the textconv attributes from the submodules and erroneously
applies the attributes defined in the superproject on the submodules'
files. The textconv cache is also saved on the superproject, even for
submodule objects.
A fix for these problems will probably require at least three changes:
- Some textconv and attributes functions (as well as their callees) will
have to be adjusted to work with arbitrary repositories. Note that
"fill_textconv()", for example, already receives a "struct repository"
but it writes the textconv cache using "write_loose_object()", which
implicitly works on "the_repository".
- grep.c functions will have to call textconv/userdiff routines passing
the "repo" field from "struct grep_source" instead of the one from
"struct grep_opt". The latter always points to "the_repository" on
"git grep" executions (see its initialization in builtin/grep.c), but
the former points to the correct repository that each source (an
object, file, or buffer) comes from.
- "userdiff_find_by_path()" might need to use a different attributes
stack for each repository it works on or reset its internal static
stack when the repository is changed throughout the calls.
For now, let's add some tests to demonstrate these problems, and also
update a NEEDSWORK comment in grep.h that mentions this bug to reference
the added tests.
Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
When repacking into a geometric series and writing a multi-pack bitmap,
it is beneficial to have the largest resulting pack be the preferred
object source in the bitmap's MIDX, since selecting the large packs can
lead to fewer broken delta chains and better compression.
Teach 'git repack' to identify this pack and pass it to the MIDX write
machinery in order to mark it as preferred.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Teach `git repack` a new `--write-midx` option for callers that wish to
persist a multi-pack index in their repository while repacking.
There are two existing alternatives to this new flag, but they don't
cover our particular use-case. These alternatives are:
- Call 'git multi-pack-index write' after running 'git repack', or
- Set 'GIT_TEST_MULTI_PACK_INDEX=1' in your environment when running
'git repack'.
The former works, but introduces a gap in bitmap coverage between
repacking and writing a new MIDX (since the repack may have deleted a
pack included in the existing MIDX, invalidating it altogether).
Setting the 'GIT_TEST_' environment variable is obviously unsupported.
In fact, even if it were supported officially, it still wouldn't work,
because it generates the MIDX *after* redundant packs have been dropped,
leading to the same issue as above.
Introduce a new option which eliminates this race by teaching `git
repack` to generate the MIDX at the critical point: after the new packs
have been written and moved into place, but before the redundant packs
have been removed.
This option is compatible with `git repack`'s '--bitmap' option (it
changes the interpretation to be: "write a bitmap corresponding to the
MIDX after one has been generated").
There is a little bit of additional noise in the patch below to avoid
repeating ourselves when selecting which packs to delete. Instead of a
single loop as before (where we iterate over 'existing_packs', decide if
a pack is worth deleting, and if so, delete it), we have two loops (the
first where we decide which ones are worth deleting, and the second
where we actually do the deleting). This makes it so we have a single
check we can make consistently when (1) telling the MIDX which packs we
want to exclude, and (2) actually unlinking the redundant packs.
There is also a tiny change to short-circuit the body of
write_midx_included_packs() when no packs remain in the case of an empty
repository. The MIDX code does not handle this, so avoid trying to
generate a MIDX covering zero packs in the first place.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
To figure out which commits we can write a bitmap for, the multi-pack
index/bitmap code does a reachability traversal, marking any commit
which can be found in the MIDX as eligible to receive a bitmap.
This approach will cause a problem when multi-pack bitmaps are able to
be generated from `git repack`, since the reference tips can change
during the repack. Even though we ignore commits that don't exist in
the MIDX (when doing a scan of the ref tips), it's possible that a
commit in the MIDX reaches something that isn't.
This can happen when a multi-pack index contains some pack which refers
to loose objects (e.g., if a pack was pushed after starting the repack
but before generating the MIDX which depends on an object which is
stored as loose in the repository, and by definition isn't included in
the multi-pack index).
By taking a snapshot of the references before we start repacking, we can
close that race window. In the above scenario (where we have a packed
object pointing at a loose one), we'll either (a) take a snapshot of the
references before seeing the packed one, or (b) take it after, at which
point we can guarantee that the loose object will be packed and included
in the MIDX.
This patch does just that. It writes a temporary "reference snapshot",
which is a list of OIDs that are at the ref tips before writing a
multi-pack bitmap. References that are "preferred" (i.e,. are a suffix
of at least one value of the 'pack.preferBitmapTips' configuration) are
marked with a special '+'.
The format is simple: one line per commit at each tip, with an optional
'+' at the beginning (for preferred references, as described above).
When provided, the reference snapshot is used to drive bitmap selection
instead of the MIDX code doing its own traversal. When it isn't
provided, the usual traversal takes place instead.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
To power a new `--write-midx` mode, `git repack` will want to write a
multi-pack index containing a certain set of packs in the repository.
This new option will be used by `git repack` to write a MIDX which
contains only the packs which will survive after the repack (that is, it
will exclude any packs which are about to be deleted).
This patch effectively exposes the function implemented in the previous
commit via the `git multi-pack-index` builtin. An alternative approach
would have been to call that function from the `git repack` builtin
directly, but this introduces awkward problems around closing and
reopening the object store, so the MIDX will be written out-of-process.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Code cleanup to limit memory consumption and tighten protocol
message parsing.
* jk/reduce-malloc-in-v2-servers:
ls-refs: reject unknown arguments
serve: reject commands used as capabilities
serve: reject bogus v2 "command=ls-refs=foo"
docs/protocol-v2: clarify some ls-refs ref-prefix details
ls-refs: ignore very long ref-prefix counts
serve: drop "keys" strvec
serve: provide "receive" function for session-id capability
serve: provide "receive" function for object-format capability
serve: add "receive" method for v2 capabilities table
serve: return capability "value" from get_capability()
serve: rename is_command() to parse_command()
