Eliminate recursion in setting/clearing marks in commit list

Eliminate recursion in setting/clearing marks in commit list

Recursion in a DAG is generally a bad idea because it could be very
deep. Be defensive and avoid recursion in mark_parents_uninteresting()
and clear_commit_marks().

mark_parents_uninteresting() learns a trick from clear_commit_marks()
to avoid malloc() in (dominant) single-parent case.

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