clean up and optimize nth_packed_object_sha1() usage

clean up and optimize nth_packed_object_sha1() usage

Let's avoid the open coded pack index reference in pack-object and use
nth_packed_object_sha1() instead.  This will help encapsulating index
format differences in one place.

And while at it there is no reason to copy SHA1's over and over while a
direct pointer to it in the index will do just fine.

(based on commit d72308e01c5977177cda0aed06cfeee9192e1247)

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