merge-recursive: Fix rename/rename(1to2) resolution for virtual merge base

merge-recursive: Fix rename/rename(1to2) resolution for virtual merge base

When renaming one file to two files, we really should be doing a content
merge.  Also, in the recursive case, undoing the renames and recording the
merged file in the index with the source of the rename (while deleting
both destinations) allows the renames to be re-detected in the
non-recursive merge and will result in fewer spurious conflicts.

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