Allow one-way tree merge to remove old files

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>