pack-objects: further work on internal rev-list logic.

pack-objects: further work on internal rev-list logic.

This teaches the internal rev-list logic to understand options
that are needed for pack handling: --all, --unpacked, and --thin.

It also moves two functions from builtin-rev-list to list-objects
so that the two programs can share more code.

Signed-off-by: Junio C Hamano <junkio@cox.net>