combine-diff: do not punt on removed or added files.

combine-diff: do not punt on removed or added files.

When we remove a file, the parents' contents are all removed so
it is not that interesting to show all of them, but the fact it
was removed when all parents had it *is* unusual.  When we add a
file, similarly the fact it was added when no parent wanted it
*is* unusual, and in addition the result matters, so show it.

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