Parent commits are usually older than their children. Thus,
on each iteration of the loop in rewrite_one, add_parents_to_list
traverses all commits previously processed by the loop.
It performs very poorly in case of very long rewrite chains.
Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>