21.5.28: Reverting a buffer leaves its status as modified

[Stephen J. Turnbull]

Ville Skyttä writes:

 > Reverting a buffer in 21.5.28 leaves the buffer's status as modified for me.

Yeah, I've reported this.  I think I have observed other operations
that should not change the modiff but do, but I can't reproduce.  I
don't know what's going on, but it goes back to at least 21.5.27
(+CVS-20070324).

Aidan and Mike are the ones to most recently touch the revert code,
but I observe the issue in builds earlier than their changes.