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

diff --git a/log-tree.c b/log-tree.c
index d3ae969..2e2be7c 100644
--- a/log-tree.c
+++ b/log-tree.c
@@ -469,6 +469,12 @@
 			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');
 		}
 	}