diff-tree: Use ---\n as a message separator

Signed-off-by: Timo Hirvonen <tihirvon@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

builtin-diff-tree.c

@@ -72,6 +72,7 @@ int cmd_diff_tree(int argc, const char **argv, char **envp)
 	init_revisions(opt);
 	opt->abbrev = 0;
 	opt->diff = 1;
+	opt->diffopt.msg_sep = "---\n";
 	argc = setup_revisions(argc, argv, opt, NULL);
 
 	while (--argc > 0) {