[PATCH] Be more like the 'resolve' strategy.

If there are non-mergeable changes leave the head contents in the
cache and update the working directory with the output from merge(1).

In the add/add and delete/modify conflict cases leave unmerged cache
entries in the index.

Signed-off-by: Fredrik Kuivinen <freku045@student.liu.se>
Signed-off-by: Junio C Hamano <junkio@cox.net>