git-pack-objects: do the delta search in reverse size order

git-pack-objects: do the delta search in reverse size order

Starting from big objects and going backwards means that we end up
picking a delta that goes from a bigger object to a smaller one.  That's
advantageous for two reasons: the bigger object is likely the newer one
(since things tend to grow, rather than shrink), and doing a delete
tends to be smaller than doing an add.

So the deltas don't tend to be top-of-tree, and the packed end result is
just slightly smaller.