rev-list --objects-edge

This new flag is similar to --objects, but causes rev-list to
show list of "uninteresting" commits that appear on the edge
commit prefixed with '-'.

Downstream pack-objects will be changed to take these as hints
to use the trees and blobs contained with them as base objects
of resulting pack, producing an incomplete (not self-contained)
pack.

Such a pack cannot be used in .git/objects/pack (it is prevented
by git-index-pack erroring out if it is fed to git-fetch-pack -k
or git-clone-pack), but would be useful when transferring only
small changes to huge blobs.

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