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>

builtin-log.c

@@ -175,6 +175,7 @@ int cmd_format_patch(int argc, const char **argv, char **envp)
 	rev.diff = 1;
 	rev.combine_merges = 0;
 	rev.ignore_merges = 1;
+	rev.diffopt.msg_sep = "---\n";
 
 	git_config(git_format_config);
 	rev.extra_headers = extra_headers;