Fix quadratic performance in rewrite_one.

Fix quadratic performance in rewrite_one.

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>