check-ignore (at least the test suite) seems to rely on the pattern
order. PATHSPEC_KEEP_ORDER is introduced to explictly express this.
The lack of PATHSPEC_MAXDEPTH_VALID is sufficient because it's the
only flag that reorders pathspecs, but it's less obvious that way.
Cc: Adam Spiers <git@adamspiers.org> Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>