emacs: Make message-summary button extend to very beginning of message.

emacs: Make message-summary button extend to very beginning of message.

There's no visible change here---we're just making the button extend
through the invisible portions of the message before the
message-summary line. The reason this is important is that it's easy
for the user to position point at the (invisible) `point-min', so we
want to ensure that there's a valid button there.