check-ignore: convert to use parse_pathspec

check-ignore: convert to use parse_pathspec

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 <redacted>
Signed-off-by: Nguyễn Thái Ngọc Duy <redacted>
Signed-off-by: Junio C Hamano <redacted>