commit_graft_pos(): take an oid instead of a bare hash

commit_graft_pos(): take an oid instead of a bare hash

All of our callers have an object_id, and are just dereferencing the
hash field to pass to us. Let's take the actual object_id instead. We
still access the hash to pass to hash_pos, but it's a step in the right
direction.

This makes the callers slightly simpler, but also gets rid of the
untyped pointer, as well as the now-inaccurate name "sha1".

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>