Allow one-way tree merge to remove old files

For some random reason (probably just because nobody noticed), the one-way
merge strategy didn't mark deleted files as deleted, so if you used

	git-read-tree -m -u <newtree>

it would update the files that got changed in the index, but it would not
delete the files that got deleted.

This should fix it, and I can't imagine that anybody depends on the old
strange "update only existing files" behaviour.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

read-tree.c

@@ -684,7 +684,7 @@ static int oneway_merge(struct cache_entry **src)
 			     merge_size);
 
 	if (!a)
-		return 0;
+		return deleted_entry(old, NULL);
 	if (old && same(old, a)) {
 		return keep_entry(old);
 	}