range-diff: avoid segfault with -I

output() reuses the same struct diff_options for multiple calls of
diff_flush().  Set the option no_free to instruct it to keep the
ignore regexes between calls and release them explicitly at the end.

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

range-diff.c

@@ -485,6 +485,7 @@ static void output(struct string_list *a, struct string_list *b,
 	else
 		diff_setup(&opts);
 
+	opts.no_free = 1;
 	if (!opts.output_format)
 		opts.output_format = DIFF_FORMAT_PATCH;
 	opts.flags.suppress_diff_headers = 1;
@@ -545,6 +546,8 @@ static void output(struct string_list *a, struct string_list *b,
 	strbuf_release(&buf);
 	strbuf_release(&dashes);
 	strbuf_release(&indent);
+	opts.no_free = 0;
+	diff_free(&opts);
 }
 
 int show_range_diff(const char *range1, const char *range2,