t6042: Add failing testcases for rename/rename/add-{source,dest} conflicts

t6042: Add failing testcases for rename/rename/add-{source,dest} conflicts

Add testcases that cover three failures with current git merge, all
involving renaming one file on both sides of history:

Case 1:
If a single file is renamed to two different filenames on different sides
of history, there should be a conflict.  Adding a new file on one of those
sides of history whose name happens to match the rename source should not
cause the merge to suddenly succeed.

Case 2:
If a single file is renamed on both sides of history but renamed
identically, there should not be a conflict.  This works fine.  However,
if one of those sides also added a new file that happened to match the
rename source, then that file should be left alone.  Currently, the
rename/rename conflict handling causes that new file to become untracked.

Case 3:
If a single file is renamed to two different filenames on different sides
of history, there should be a conflict.  This works currently.  However,
if those renames also involve rename/add conflicts (i.e. there are new
files on one side of history that match the destination of the rename of
the other side of history), then the resulting conflict should be recorded
in the index, showing that there were multiple files with a given filename.
Currently, git silently discards one of file versions.

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