pack-objects: get rid of create_final_object_list()

pack-objects: get rid of create_final_object_list()

Because we don't have to know the SHA1 h(hence the name) of the pack
up front anymore, let's get rid of yet another global sorted object list
and sort them only in write_index_file(), then compute the object list
SHA1 on the fly.

This has the advantage of saving another chunk of memory, and the sorted
list SHA1 won't be computed needlessly on servers during a fetch.

Of course the cunning plan is also to make write_index_file() much like
the function with the same name in index-pack.c for an eventual easy
sharing.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>