Hello, I am using GNU Emacs 23.0.50.1 (i686-pc-linux-gnu, X toolkit,
 Xaw3d scroll bars) of 2007-12-05

/etc/TODO says: revert-buffer should eliminate overlays and the mark.

This patch should do that. It removes the mark, mark-ring and all
overlays after `before-revert-hook' is run and before
`after-revert-hook' is run.

Additionally, the patch changes the doc-string to be more explicit about
how other modes use revert-buffer via 'revert-buffer-function'.