use commit_stack instead of prio_queue in LIFO mode

use commit_stack instead of prio_queue in LIFO mode

A prio_queue with a NULL compare function acts as a stack -- the last
element in is the first one out (LIFO).  Use an actual commit_stack
instead where possible, as it documents the behavior better, provides
type safety and saves some memory because prio_queue stores an
additional tie-breaking counter per element.

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