Minimal BSDDB storage which does not override abortVersion() and
commitVersion()).

commitVersion() can't be an alias for abortVersion() because it takes
a different number of arguments.  Provide a simple implementation.

Changed "from POSException import UndoError" to "import POSException"
and use POSException.UndoError in undo().  This is because
abortVersion() and commitVersion() also reference exceptions in the
POSException module.

Aside: fix __version__ extraction from $Revision$ to be portable in
the face of "cvs co -kv".