This is important for chaining "undo" for a given object in a single
transaction, to allow checking for conflicts.
Also, this means that get_baseTID return value cannot be used to update
cache when undoing, so only update cache when storing a new revision.
Actually, cache was never updated when undoing anyway, as current snapshot
TID was unlikely to exist as a serial for any object, so it would not be
found in cache and update would be a no-op.

git-svn-id: https://svn.erp5.org/repos/neo/trunk@2597 71dcc9de-d417-0410-9af5-da40c76e7ee4