push: flag updates that require force

Add a flag for indicating an update to a reference requires force.
Currently the `nonfastforward` flag is used for this when generating the
status message.  A separate flag insulates dependent logic from the
details of set_ref_status_for_push().

Signed-off-by: Chris Rorvick <chris@rorvick.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -999,7 +999,9 @@ struct ref {
 	unsigned char old_sha1[20];
 	unsigned char new_sha1[20];
 	char *symref;
-	unsigned int force:1,
+	unsigned int
+		force:1,
+		requires_force:1,
 		merge:1,
 		nonfastforward:1,
 		not_forwardable:1,