While it makes sense to allow merging unrelated histories of two
projects that started independently into one, in the way "gitk" was
merged to "git" itself aka "the coolest merge ever", such a merge is
still an unusual event. Worse, if somebody creates an independent
history by starting from a tarball of an established project and
sends a pull request to the original project, "git merge" however
happily creates such a merge without any sign of something unusual
is happening.
Teach "git merge" to refuse to create such a merge by default,
unless the user passes a new "--allow-unrelated-histories" option to
tell it that the user is aware that two unrelated projects are
merged.
Because such a "two project merge" is a rare event, a configuration
option to always allow such a merge is not added.
We could add the same option to "git pull" and have it passed
through to underlying "git merge". I do not have a fundamental
opposition against such a feature, but this commit does not do so
and instead leaves it as low-hanging fruit for others, because such
a "two project merge" would be done after fetching the other project
into some location in the working tree of an existing project and
making sure how well they fit together, it is sufficient to allow a
local merge without such an option pass-through from "git pull" to
"git merge". Many tests that are updated by this patch does the
pass-through manually by turning:
git pull something
into its equivalent:
git fetch something &&
git merge --allow-unrelated-histories FETCH_HEAD
If somebody is inclined to add such an option, updated tests in this
change need to be adjusted back to:
git pull --allow-unrelated-histories something
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The collect_parents() function now is responsible for
1. parsing the commits given on the command line into a list of
commits to be merged;
2. filtering these parents into independent ones; and
3. optionally calling fmt_merge_msg() via prepare_merge_message()
to prepare an auto-generated merge log message, using fake
contents that FETCH_HEAD would have had if these commits were
fetched from the current repository with "git pull . $args..."
Make "git merge FETCH_HEAD" to be the same as the traditional
git merge "$(git fmt-merge-msg <.git/FETCH_HEAD)" $commits
invocation of the command in "git pull", where $commits are the ones
that appear in FETCH_HEAD that are not marked as not-for-merge, by
making it do a bit more, specifically:
- noticing "FETCH_HEAD" is the only "commit" on the command line
and picking the commits that are not marked as not-for-merge as
the list of commits to be merged (substitute for step #1 above);
- letting the resulting list fed to step #2 above;
- doing the step #3 above, using the contents of the FETCH_HEAD
instead of fake contents crafted from the list of commits parsed
in the step #1 above.
Note that this changes the semantics. "git merge FETCH_HEAD" has
always behaved as if the first commit in the FETCH_HEAD file were
directly specified on the command line, creating a two-way merge
whose auto-generated merge log said "merge commit xyz". With this
change, if the previous fetch was to grab multiple branches (e.g.
"git fetch $there topic-a topic-b"), the new world order is to
create an octopus, behaving as if "git pull $there topic-a topic-b"
were run. This is a deliberate change to make that happen, and
can be seen in the changes to t3033 tests.
Signed-off-by: Junio C Hamano <gitster@pobox.com>
We seem to have tests for specific merge strategy backends
(e.g. recursive), but not much test coverage for the "git merge"
itself. As I am planning to update the semantics of merging
"FETCH_HEAD" in such a way that these two
git pull . topic_a topic_b...
vs.
git fetch . topic_a topic_b...
git merge FETCH_HEAD
are truly equivalent, let me add a few test cases to cover the
tricky ones.
Signed-off-by: Junio C Hamano <gitster@pobox.com>
