Change pack_date interpretation?

[d-maurer]

Currently, FileStorage.pack deletes non current states when they have been created before pack_date (the parameter is aktually named t, a timestamp).

While this leads to an easy implementation, it does not conform to the intuitive interpretation: I have the complete history up to pack_date (and can revert up to this date). For this, we would need one additional older state.

With the current interpretation, even a very old pack_date does not guarantee that a very recent modification (to a very stable object) can be reverted.
Should we change the pack_date itnerpretation to include one additional older state?