t6423: update directory rename detection tests with new rule

t6423: update directory rename detection tests with new rule

While investigating the issues highlighted by the testcase in the
previous patch, I also found a shortcoming in the directory rename
detection rules.  Split testcase 6b into two to explain this issue
and update directory-rename-detection.txt to remove one of the previous
rules that I know believe to be detrimental.  Also, update the wording
around testcase 8e; while we are not modifying the results of that
testcase, we were previously unsure of the appropriate resolution of
that test and the new rule makes the previously chosen resolution for
that testcase a bit more solid.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>