combine-diff: diff-files fix.

combine-diff: diff-files fix.

When showing a conflicted merge from index stages and working
tree file, we did not fetch the mode from the working tree,
and mistook that as a deleted file.  Also if the manual
resolution (or automated resolution by git rerere) ended up
taking either parent's version, we did not show _anything_ for
that path.  Either was quite bad and confusing.

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