ls-files -i: micro-optimize path_excluded()

ls-files -i: micro-optimize path_excluded()

As we know a caller that does not recurse is calling us in the index
order, we can remember the last directory we found to be excluded
and see if the path we are looking at is still inside it, in which
case we can just answer that it is excluded.

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