Make regenerated worlds not delete their previous counterpart's rollbacks

[cgaldieri]

I am relatively new to the game, due to this, sometimes when big mistakes happen like when my fully tamed beefalo dies, i rollback my game to not have to waste time doing it all again, however, it's way too easy to accidentally regenerate a world instead of rolling back, i guess the reason why you lose your rollbacks is for when you willingly decide to do it and to not waste storage space.