ls-tree: major rewrite to do pathspec

ls-tree: major rewrite to do pathspec

git-ls-tree should be rewritten to use a pathspec the same way everybody
else does. Right now it's the odd man out: if you do

	git-ls-tree HEAD divers/char drivers/

it will show the same files _twice_, which is not how pathspecs in general
work.

How about this patch? It breaks some of the git-ls-tree tests, but it
makes git-ls-tree work a lot more like other git pathspec commands, and it
removes more than 150 lines by re-using the recursive tree traversal (but
the "-d" flag is gone for good, so I'm not pushing this too hard).

		Linus