t6042: Add tests for content issues with modify/rename/directory conflicts

t6042: Add tests for content issues with modify/rename/directory conflicts

Add testcases that cover a variety of merge issues with files being
renamed and modified on different sides of history, when there are
directories possibly conflicting with the rename location.

Case 1:
On one side of history, a file is modified and a new directory is added.
On the other side of history, the file is modified in a non-conflicting
way but is renamed to the location of the new directory.

Case 2:
[Same as case 1, but there is also a content conflict.  In detail:]
On one side of history, a file is modified and a new directory is added.
On the other side of history, the file is modified in a conflicting way
and it is renamed to the location of the new directory.

Case 3:
[Similar to case 1, but the "conflicting" directory is the directory
where the file original resided.  In detail:]
On one side of history, a file is modified.  On the other side of history,
the file is modified in a non-conflicting way, but the directory it was
under is removed and the file is renamed to the location of the directory
it used to reside in (i.e. 'sub/file' gets renamed to 'sub').  This is
flagged as a directory/rename conflict, but should be able to be resolved
since the directory can be cleanly removed by the merge.

One branch renames a file and makes a file where the directory the renamed
file used to be in, and the other branch updates the file in
place. Merging them should resolve it cleanly as long as the content level
change on the branches do not overlap and rename is detected, or should
leave conflict without losing information.

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