refs: convert delete_ref and refs_delete_ref to struct object_id

Convert delete_ref and refs_delete_ref to take a pointer to struct
object_id.  Update the documentation accordingly, including referring to
null_oid in lowercase, as it is not a #define constant.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/reset.c b/builtin/reset.c
index 9cd89b2..5f3632e 100644
--- a/builtin/reset.c
+++ b/builtin/reset.c
@@ -269,7 +269,7 @@ static int reset_refs(const char *rev, const struct object_id *oid)
 		update_ref_oid(msg.buf, "ORIG_HEAD", orig, old_orig, 0,
 			   UPDATE_REFS_MSG_ON_ERR);
 	} else if (old_orig)
-		delete_ref(NULL, "ORIG_HEAD", old_orig->hash, 0);
+		delete_ref(NULL, "ORIG_HEAD", old_orig, 0);
 	set_reflog_message(&msg, "updating HEAD", rev);
 	update_ref_status = update_ref_oid(msg.buf, "HEAD", oid, orig, 0,
 				       UPDATE_REFS_MSG_ON_ERR);