bug#1713: follow-mode with three vertical windows causes Emacs to hang

[Teemu Likonen]

Emacs hangs when follow-mode is used with three vertically split windows
and the buffer/window text is scrolled. The "hang" means that Emacs
stops responding and takes nearly all the CPU time.

Steps to reproduce:

    $ emacs -Q

    C-h r           (Emacs manual)
    C-x 3
    C-x 3
    C-x +
    M-x follow-mode RET

Now press C-n (next-line) down and keep it down. The cursor moves and
the buffer text scrolls but at some point Emacs just hangs.

This is always reproducible with my Emacs 22.2.1 (Debian package) and
Emacs 23 (http://emacs.orebokech.com/).