graph.c: register a callback for graph output

It will look better if the 'git log --graph' print
the graph pading lines before the diff output just
like what it does for commit message.
And this patch leverage the new diff prefix callback
function to achieve this.

Signed-off-by: Bo Yang <struggleyb.nku@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

graph.c

@@ -211,6 +211,18 @@ struct git_graph {
 	unsigned short default_column_color;
 };
 
+static struct strbuf *diff_output_prefix_callback(struct diff_options *opt, void *data)
+{
+	struct git_graph *graph = data;
+	static struct strbuf msgbuf = STRBUF_INIT;
+
+	assert(graph);
+
+	strbuf_reset(&msgbuf);
+	graph_padding_line(graph, &msgbuf);
+	return &msgbuf;
+}
+
 struct git_graph *graph_init(struct rev_info *opt)
 {
 	struct git_graph *graph = xmalloc(sizeof(struct git_graph));
@@ -244,6 +256,13 @@ struct git_graph *graph_init(struct rev_info *opt)
 	graph->mapping = xmalloc(sizeof(int) * 2 * graph->column_capacity);
 	graph->new_mapping = xmalloc(sizeof(int) * 2 * graph->column_capacity);
 
+	/*
+	 * The diff output prefix callback, with this we can make
+	 * all the diff output to align with the graph lines.
+	 */
+	opt->diffopt.output_prefix = diff_output_prefix_callback;
+	opt->diffopt.output_prefix_data = graph;
+
 	return graph;
 }