builtin/pack-objects: convert to struct object_id

This is one of the last unconverted callers to peel_ref.  While we're
fixing that, convert the rest of the file, since it will need to be
converted at some point anyway.

Signed-off-by: brian m. carlson <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: bk2204