Output the graph columns at the end of the commit message

There is an empty line between the commit message and the diff
output. Add the graph columns as prefix of this line.

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

log-tree.c

@@ -469,6 +469,12 @@ int log_tree_diff_flush(struct rev_info *opt)
 			int pch = DIFF_FORMAT_DIFFSTAT | DIFF_FORMAT_PATCH;
 			if ((pch & opt->diffopt.output_format) == pch)
 				printf("---");
+			if (opt->diffopt.output_prefix) {
+				struct strbuf *msg = NULL;
+				msg = opt->diffopt.output_prefix(&opt->diffopt,
+					opt->diffopt.output_prefix_data);
+				fwrite(msg->buf, msg->len, 1, stdout);
+			}
 			putchar('\n');
 		}
 	}