refs: convert read_ref and read_ref_full to object_id

All but two of the call sites already have parameters using the hash
parameter of struct object_id, so convert them to take a pointer to the
struct directly.  Also convert refs_read_refs_full, the underlying
implementation.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/notes-utils.c b/notes-utils.c
index 3a70089..5c8e70c 100644
--- a/notes-utils.c
+++ b/notes-utils.c
@@ -18,7 +18,7 @@ void create_notes_commit(struct notes_tree *t, struct commit_list *parents,
 	if (!parents) {
 		/* Deduce parent commit from t->ref */
 		struct object_id parent_oid;
-		if (!read_ref(t->ref, parent_oid.hash)) {
+		if (!read_ref(t->ref, &parent_oid)) {
 			struct commit *parent = lookup_commit(&parent_oid);
 			if (parse_commit(parent))
 				die("Failed to find/parse commit %s", t->ref);