Thin pack - create packfile with missing delta base.

This goes together with "rev-list --object-edge" change, to feed
pack-objects list of edge commits in addition to the usual
object list.  Upon seeing such list, pack-objects loosens the
usual "self contained delta" constraints, and can produce delta
against blobs and trees contained in the edge commits without
storing the delta base objects themselves.

The resulting packfile is not usable in .git/object/packs, but
is a good way to implement "delta-only" transfer.

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