Feature request: Accidental rollback protection

[claraphyll]

It would be nice if deploy-rs could detect that the configuration running on the target host is newer than the one being deployed. This could be done by checking if the self.rev or self.dirtyRev of the configuration deployed on the host is an ancestor of the current self.rev or self.dirtyRev e. g. using git merge-base --is-ancestor <commit1> <commit2>.

There should be a flag to override this behavior if a rollback is desired.

I can't really think of a way to implement this using checks, since it requires checking the target host.