for_each_object_in_pack(): clarify pack vs index ordering

for_each_object_in_pack(): clarify pack vs index ordering

We may return objects in one of two orders: how they appear in the .idx
(sorted by object id) or how they appear in the packfile itself. To
further complicate matters, we have two ordering variables, "i" and
"pos", and it is not clear to which order they apply.

Let's clarify this by using an unambiguous name where possible, and
leaving a comment for the variable that does double-duty.

Signed-off-by: Jeff King <peff@peff.net>
Acked-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>