sqlitestore: add an `ancestors` method We will need it during bundling. The implementation mirror the one in revlog.

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.