revision: export commit_stack

revision: export commit_stack

Dynamic arrays of commit pointers are used in several places.  Some of
them use a custom struct to hold array, item count and capacity, others
have them as separate variables linked by a common name part.

Pick one succinct, clean implementation -- commit_stack -- and convert
the different variants to it to reduce code duplication.

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