Add msg_sep to diff_options

Add msg_sep variable to struct diff_options.  msg_sep is printed after
commit message.  Default is "\n", format-patch sets it to "---\n".

This also removes the second argument from show_log() because all
callers derived it from the first argument:

    show_log(rev, rev->loginfo, ...

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

diff.c

@@ -1424,6 +1424,7 @@ void diff_setup(struct diff_options *options)
 	options->break_opt = -1;
 	options->rename_limit = -1;
 	options->context = 3;
+	options->msg_sep = "\n";
 
 	options->change = diff_change;
 	options->add_remove = diff_addremove;