t6046: testcases checking whether updates can be skipped in a merge
Add several tests checking whether updates can be skipped in a merge.
Also add several similar testcases for where updates cannot be skipped in
a merge to make sure that we skip if and only if we should.
In particular:
* Testcase 1a (particularly 1a-check-L) would have pointed out the
problem Linus has been dealing with for year with his merges[1].
* Testcase 2a (particularly 2a-check-L) would have pointed out the
problem with my directory-rename-series before it broke master[2].
* Testcases 3[ab] (particularly 3a-check-L) provide a simpler testcase
than 12b of t6043 making that one easier to understand.
* There are several complementary testcases to make sure we're not just
fixing those particular issues while regressing in the opposite
direction.
* There are also a pair of tests for the special case when a merge
results in a skippable update AND the user has dirty modifications to
the path.
[1] https://public-inbox.org/git/CA+55aFzLZ3UkG5svqZwSnhNk75=fXJRkvU1m_RHBG54NOoaZPA@mail.gmail.com/
[2] https://public-inbox.org/git/xmqqmuya43cs.fsf@gitster-ct.c.googlers.com/
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>