rebase should handle idempotency

Derrick got an "old and new refs are equal" on rebase error; this is similar to OCPBUGS-1899 but I think has a different root cause. In this case, when a manual rollback is performed via the bootloader, we've computed that there's an osimageurl diff between the expected and desired state, but actually the desired state is already set.

We just need to skip doing the rebase if we're already in the target state.

(A real root of this problem again is that the whole "current/desired config" thing is trying to track state independently of the bootloader...if we made node state == container image, all of that goes away. The MCO would understand that it got booted into a previous state)