pathspec: add match_pathspec_depth()

pathspec: add match_pathspec_depth()

match_pathspec_depth() is a clone of match_pathspec() except that it
can take depth limit. Computation is a bit lighter compared to
match_pathspec() because it's usually precomputed and stored in struct
pathspec.

In long term, match_pathspec() and match_one() should be removed in
favor of this function.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>