t6600: add test for merge-base early exit with clock skew

t6600: add test for merge-base early exit with clock skew

Add a topology where the correct merge base (M2) has a lower
committer date than its ancestor (M1) due to clock skew.  With a
v1 commit graph (topological levels only, no corrected commit
dates), paint_down_to_common() falls back to commit-date ordering.
In that mode, M1 pops before M2, acquires both paint sides, and
the !FIND_ALL early exit fires -- returning the wrong merge base.

Mark the test as test_expect_failure to document the bug; the next
commit will fix it.

Signed-off-by: Kristofer Karlsson <krka@spotify.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>