Add a method to address snapshots relative to the latest / general relative snapshot adressing

[Cookie04DE]

It would be very useful to be able to address snapshots not just with latest but also with something like latest-1, for example when diffing a file to figure out what happened between the current and previous snapshot.

Sort of like how to address git commits with HEAD and HEAD~1.

In a similar vain something like this with ids would be useful too, so one can address a snapshot and it's parent snapshots.

Maybe even in the positive ("future") direction, but I am not familiar enough with the code to know if that is feasible.