Make sure we generate the whole commit list before trying to sort it topologically

This was my cherry-pickng merge bug.  But topo-order still shows strange
behaviour with multiple heads, so keep gitk using --merge-order for now.

rev-list.c

@@ -473,6 +473,7 @@ int main(int argc, char **argv)
 		}
 		if (!strcmp(arg, "--topo-order")) {
 		        topo_order = 1;
+		        limited = 1;
 			continue;
 		}