log --graph: fix break in graph lines

Output from "git log --graph --stat -p" broke the ancestry graph lines
with a single empty line between the diffstat and the patch.

Signed-off-by: Lucian Poston <lucian.poston@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -4266,6 +4266,12 @@ void diff_flush(struct diff_options *options)
 
 	if (output_format & DIFF_FORMAT_PATCH) {
 		if (separator) {
+			if (options->output_prefix) {
+				struct strbuf *msg = NULL;
+				msg = options->output_prefix(options,
+					options->output_prefix_data);
+				fwrite(msg->buf, msg->len, 1, stdout);
+			}
 			putc(options->line_termination, options->file);
 			if (options->stat_sep) {
 				/* attach patch instead of inline */