transport-helper: die early on encountering deleted refs

Remote helpers do not support deleting refs by means of the 'export'
command sincethe fast-import protocol does not support it.

Check explicitly for deleted refs and die early.

Signed-off-by: Sverre Rabbelier <srabbelier@gmail.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

transport-helper.c

@@ -746,6 +746,10 @@ static int push_refs_with_export(struct transport *transport,
 		}
 		free(private);
 
+		if (ref->deletion) {
+			die("remote-helpers do not support ref deletion");
+		}
+
 		if (ref->peer_ref)
 			string_list_append(&revlist_args, ref->peer_ref->name);