pack-objects: thin pack micro-optimization.

pack-objects: thin pack micro-optimization.

Since we sort objects by type, hash, preferredness and then
size, after we have a delta against preferred base, there is no
point trying a delta with non-preferred base.  This seems to
save expensive calls to diff-delta and it also seems to save the
output space as well.

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