Add msg_sep to diff_options

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>