graph: use SWAP macro

Exchange the values of graph->columns and graph->new_columns using the
macro SWAP instead of hand-rolled code.  The result is shorter and
easier to read.

This transformation was not done by the semantic patch swap.cocci
because there's an unrelated statement between the second and the last
step of the exchange, so it didn't match the expected pattern.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

graph.c

@@ -463,7 +463,6 @@ static void graph_update_width(struct git_graph *graph,
 static void graph_update_columns(struct git_graph *graph)
 {
 	struct commit_list *parent;
-	struct column *tmp_columns;
 	int max_new_columns;
 	int mapping_idx;
 	int i, seen_this, is_commit_in_columns;
@@ -476,11 +475,8 @@ static void graph_update_columns(struct git_graph *graph)
 	 * We'll re-use the old columns array as storage to compute the new
 	 * columns list for the commit after this one.
 	 */
-	tmp_columns = graph->columns;
-	graph->columns = graph->new_columns;
+	SWAP(graph->columns, graph->new_columns);
 	graph->num_columns = graph->num_new_columns;
-
-	graph->new_columns = tmp_columns;
 	graph->num_new_columns = 0;
 
 	/*