graph.c: infinite loop in git whatchanged --graph -m

Running "whatchanged --graph -m" on a simple two-head merges
can fall into infinite loop.

Signed-off-by: Michał Kiedrowicz <michal.kiedrowicz@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/api-history-graph.txt

@@ -33,11 +33,11 @@ The following utility functions are wrappers around `graph_next_line()` and
 They can all be called with a NULL graph argument, in which case no graph
 output will be printed.
 
-* `graph_show_commit()` calls `graph_next_line()` until it returns non-zero.
-  This prints all graph lines up to, and including, the line containing this
-  commit.  Output is printed to stdout.  The last line printed does not contain
-  a terminating newline.  This should not be called if the commit line has
-  already been printed, or it will loop forever.
+* `graph_show_commit()` calls `graph_next_line()` and
+  `graph_is_commit_finished()` until one of them return non-zero.  This prints
+  all graph lines up to, and including, the line containing this commit.
+  Output is printed to stdout.  The last line printed does not contain a
+  terminating newline.
 
 * `graph_show_oneline()` calls `graph_next_line()` and prints the result to
   stdout.  The line printed does not contain a terminating newline.

graph.c

@@ -1199,7 +1199,7 @@ void graph_show_commit(struct git_graph *graph)
 	if (!graph)
 		return;
 
-	while (!shown_commit_line) {
+	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);
 		if (!shown_commit_line)

t/t4202-log.sh

@@ -264,6 +264,16 @@ test_expect_success 'log --graph with merge' '
 	test_cmp expect actual
 '
 
+test_expect_success 'log --raw --graph -m with merge' '
+	git log --raw --graph --oneline -m master | head -n 500 >actual &&
+	grep "initial" actual
+'
+
+test_expect_success 'diff-tree --graph' '
+	git diff-tree --graph master^ | head -n 500 >actual &&
+	grep "one" actual
+'
+
 cat > expect <<\EOF
 *   commit master
 |\  Merge: A B