[feature] implement umoci-rebase

[cyphar]

I was working on something like this for Docker but the implementation of DiffIDs at the time was incredibly confusing (and I was working with the dockerv1 image specification that was mostly undocumented). umoci rebase should be fairly easy to implement (and if we implemented it like git we could also make it easier to rewrite and squash history) but the annoying thing is going to be implementing the conflict resolution.