patch to autorevert.el

[bigfaceworm]

This patch adds a new hook to be run whenever a buffer is reverted.
Original autorevert.el was distributed with Emacs 21.1

------------------------------------------------
130a131,135
> (defcustom auto-revert-buffer-reverted-hook nil
>   "Functions to run when a buffer is reverted."
>   :group 'auto-revert
>   :type 'hook)
>
345c350,351
<               (revert-buffer t t t)))
---
>                 (run-hooks 'auto-revert-buffer-reverted-hook)
>                 (revert-buffer t t t)))