emitrevision: if we need to compute a delta on the fly, try p1 or p2 first Falling back to `prev` does not yield any real value on modern storage and result in pathological changes to be created on the other side. Doing a delta against a parent will likely be smaller (helping the network) and will be safer to apply on the client (helping future pulls by Triggering intermediate snapshop where they will be needed by later deltas).

emitrevision: simplify the fallback to computed delta Not using the stored delta, or having a full snapshot on disk behave the same ways, so lets use the same code path for that, this is simpler, and it update will be simpler.

emitrevision: also check the parents in the availability closure One of the point of having a closure is to gather the logic in it. So we gather the logic. The `parents[:]` part is a bit ugly but will be replaced by better code soon anyway.

emitrevision: add a small closure to check if a base is usable We will make more use of this and make it more complex too.