ls-tree: --name-only

Fingers of some "git diff" users are trained to do --name-only
which git-ls-tree unfortunately does not take.  With this,

	cd sub/directory && git-ls-tree -r --name-only ..

would show only the names not object names nor modes.  I threw
in another synonym --name-status only for usability, but
obviously ls-tree does not do any comparison so what it does is
the same as --name-only.

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