ref_newer: convert to use struct object_id

Convert ref_newer and its caller to use struct object_id instead of
unsigned char *.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Jeff King <peff@peff.net>
diff --git a/builtin/remote.c b/builtin/remote.c
index 68dec16..6694cf2 100644
--- a/builtin/remote.c
+++ b/builtin/remote.c
@@ -417,7 +417,7 @@
 		else if (is_null_oid(&ref->old_oid))
 			info->status = PUSH_STATUS_CREATE;
 		else if (has_object_file(&ref->old_oid) &&
-			 ref_newer(ref->new_oid.hash, ref->old_oid.hash))
+			 ref_newer(&ref->new_oid, &ref->old_oid))
 			info->status = PUSH_STATUS_FASTFORWARD;
 		else
 			info->status = PUSH_STATUS_OUTOFDATE;