pack-objects: equal objects in size should delta against newer objects

pack-objects: equal objects in size should delta against newer objects

Before finding best delta combinations, we sort objects by name hash,
then by size, then by their position in memory.  Then we walk the list
backwards to test delta candidates.

We hope that a bigger size usually means a newer objects.  But a bigger
address in memory does not mean a newer object.  So the last comparison
must be reversed.

Signed-off-by: Nicolas Pitre <redacted>
Signed-off-by: Junio C Hamano <redacted>