t6423: add tests of dual directory rename plus add/add conflict

t6423: add tests of dual directory rename plus add/add conflict

This is an attempt at minimalizing a testcase reported by Glen Choo
with tensorflow where merge-ort would report an assertion failure:

    Assertion failed: (ci->filemask == 2 || ci->filemask == 4), function apply_directory_rename_modifications, file merge-ort.c, line 2410

reversing the direction of the merge provides a different error:

    error: cache entry has null sha1: ...
    fatal: unable to write .git/index

so we add testcases for both.  With these new testcases, the
recursive strategy differs in that it returns the latter error for
both merge directions.

These testcases are somehow a little different than Glen's original
tensorflow testcase in that these ones trigger a bug with the recursive
algorithm whereas his testcase didn't.  I figure that means these
testcases somehow manage to be more comprehensive.

Reported-by: Glen Choo <redacted>
Signed-off-by: Elijah Newren <redacted>
Signed-off-by: Junio C Hamano <redacted>