Convert peel_ref (and its corresponding backend) to struct object_id.

This transformation was done with an update to the declaration,
definition, comments, and test helper and the following semantic patch:

@@
expression E1, E2;
@@
- peel_ref(E1, E2.hash)
+ peel_ref(E1, &E2)

@@
expression E1, E2;
@@
- peel_ref(E1, E2->hash)
+ peel_ref(E1, E2)
Signed-off-by: Nbrian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>