t6416: correct expectation for rename/rename(1to2) + directory/file

t6416: correct expectation for rename/rename(1to2) + directory/file

When files are renamed and modified, we need to do three-way content
merges to get the appropriate content in the right location.  When we
have a rename/rename(1to2) conflict (both sides rename the same file,
but differently), that merged content should be placed in each of the
two resulting files.  merge-recursive handled that fine when that was
all that was involved, but when one or more of the two resulting files
were ALSO involved in a directory/file conflict, it failed to propagate
the merged content to that file.  Unfortunately, the one test in t6416
that touched on this combination of cases had been coded to not expect
the merged contents to be present.

Fix the test to check for the right behavior, and record how the
different merge backends will be expected to handle it.

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