log: fix memory leak if --graph is passed multiple times

Signed-off-by: Alex Henrie <alexhenrie24@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

graph.c

@@ -401,6 +401,18 @@ struct git_graph *graph_init(struct rev_info *opt)
 	return graph;
 }
 
+void graph_clear(struct git_graph *graph)
+{
+	if (!graph)
+		return;
+
+	free(graph->columns);
+	free(graph->new_columns);
+	free(graph->mapping);
+	free(graph->old_mapping);
+	free(graph);
+}
+
 static void graph_update_state(struct git_graph *graph, enum graph_state s)
 {
 	graph->prev_state = graph->state;

graph.h

@@ -139,6 +139,11 @@ void graph_set_column_colors(const char **colors, unsigned short colors_max);
  */
 struct git_graph *graph_init(struct rev_info *opt);
 
+/*
+ * Free a struct git_graph.
+ */
+void graph_clear(struct git_graph *graph);
+
 /*
  * Update a git_graph with a new commit.
  * This will cause the graph to begin outputting lines for the new commit

revision.c

@@ -2426,6 +2426,7 @@ static int handle_revision_opt(struct rev_info *revs, int argc, const char **arg
 	} else if (!strcmp(arg, "--graph")) {
 		revs->topo_order = 1;
 		revs->rewrite_parents = 1;
+		graph_clear(revs->graph);
 		revs->graph = graph_init(revs);
 	} else if (!strcmp(arg, "--encode-email-headers")) {
 		revs->encode_email_headers = 1;