built-in tar-tree and remote tar-tree

This makes tar-tree a built-in.  As an added bonus, you can now
say:

	git tar-tree --remote=remote-repository <ent> [<base>]

This does not work with git-daemon yet, but should work with
localhost and git over ssh transports.

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