unpack-trees.c: use path_excluded() in check_ok_to_remove()

unpack-trees.c: use path_excluded() in check_ok_to_remove()

This function is responsible for determining if a path that is not
tracked is ignored and allow "checkout" to overwrite it as needed.
It used excluded() without checking if higher level directory in the
path is ignored; correct it to use path_excluded() for this check.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
---

 * There are uses of lower-level interface excluded_from_list() in
   the codepath for narrow-checkout hack; they are supposed to be
   already checking each level as they descend, and are not touched
   with this patch.