rollback using helm ends with an error if "last" is used

Hi! Thanks for the great tool. We're started using tern with a k8s job that is recreated with each new helm release. That also means that a previous version of the job is created with each helm rollback.
For simplicity we run tern with "last" destination. That greatly simplifies experience.

This piece of code is causing problems with rollback. https://github.com/jackc/tern/blob/22a33fd044cac3cde941b109580223c073c6302b/migrate/migrate.go#L303-L306

I suggest having a CLI argument to turn this control off if flag is set. WDYT?

Start in migrate/migrate.go around lines 303-306 and trace the rollback path when the destination is "last". Determine how the Helm rollback scenario reaches the error and how a CLI option should control that check. Done means the selected option avoids the rollback error without changing the default behavior.

Written by the indexing model from the issue text.