This fixes the inability to undo T-1 after undoing T.
For reference, this is what is done in RelStorage. It is not as restrictive
as FileStorage, but implementing the same support would/will take more
time.

git-svn-id: https://svn.erp5.org/repos/neo/trunk@1877 71dcc9de-d417-0410-9af5-da40c76e7ee4