pack-objects: reverse the delta search sort list

pack-objects: reverse the delta search sort list

It is currently sorted and then walked backward.  Not only this doesn't
feel natural for my poor brain, but it would make the next patch less
obvious as well.

So reverse the sort order, and reverse the list walking direction,
which effectively produce the exact same end result as before.

Also bring the relevant comment nearer the actual code and adjust it
accordingly, with minor additional clarifications.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>