commit-reach: avoid commit_list_insert_by_date()

commit-reach: avoid commit_list_insert_by_date()

Building a list using commit_list_insert_by_date() has quadratic worst
case complexity.  Avoid it by just appending in the loop and sorting at
the end.

The number of merge bases is usually small, so don't expect speedups in
normal repositories.  It has no limit, though.  The added perf test
shows a nice improvement when dealing with 16384 merge bases:

Test                     v2.51.1           HEAD
-----------------------------------------------------------------
6010.2: git merge-base   0.55(0.54+0.00)   0.03(0.02+0.00) -94.5%

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>