graph: output padding for merge subsequent parents

When showing merges in git-log, the same commit is shown once for each
parent.  Combined with "--graph" this results in graph_show_commit()
being called once for each parent without graph_update() being called.

Currently graph_show_commit() does not print anything on subsequent
invocations for the same commit (this was changed by commit 656197a -
"graph.c: infinite loop in git whatchanged --graph -m" from the previous
behaviour of looping infinitely).

Change this so that if the graph code believes it has already shown the
commit it prints a single padding line.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

graph.c

@@ -1227,6 +1227,16 @@ void graph_show_commit(struct git_graph *graph)
 	if (!graph)
 		return;
 
+	/*
+	 * When showing a diff of a merge against each of its parents, we
+	 * are called once for each parent without graph_update having been
+	 * called.  In this case, simply output a single padding line.
+	 */
+	if (graph_is_commit_finished(graph)) {
+		graph_show_padding(graph);
+		shown_commit_line = 1;
+	}
+
 	while (!shown_commit_line && !graph_is_commit_finished(graph)) {
 		shown_commit_line = graph_next_line(graph, &msgbuf);
 		fwrite(msgbuf.buf, sizeof(char), msgbuf.len, stdout);