Request for new command

[Howard D. Trachtman <HDT@MIT-OZ>]

  I'd like to see a command that is a combination between revert file
and undo.   What I would like it to do is revert the buffer the state
it was in at the last reading or writing of the buffer via the undo commands,
and if it is not able to do so, then to tell me and offer to do meta-x Revert File.