diff.c: do not use pathname comparison to tell renames

diff.c: do not use pathname comparison to tell renames

The final output from diff used to compare pathnames between
preimage and postimage to tell if the filepair is a rename/copy.
By explicitly marking the filepair created by diffcore_rename(),
the output routine, resolve_rename_copy(), does not have to do
so anymore.  This helps feeding a filepair that has different
pathnames in one and two elements to the diff machinery (most
notably, comparing two blobs).

Signed-off-by: Junio C Hamano <junkio@cox.net>