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>

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