Skipping history and save bandwidth: how can I jump between shallow clones, without sending unused blobs

I think this is encapsulated in the v2/promisor work, but wanted to check how close that was to fruition, and that it would indeed be possible.

This would enable replacement of any workflow that presently uses rsync to update.

If I have a clone (ideally shallow already) at A, I'd like to update it to the latest remote tip at F, ALSO at depth 1, without fetching the intermediate history (B..E), or blobs unique to the intermediate history [and no longer referenced from any tree at F].

I critically want to ensure:
- that the remote does NOT send any blobs that I already have.
- that the remote does NOT send any blobs that are not reachable from
  the new tip (e.g. blobs that existed between the old state and the new
  state, but aren't needed anymore).