revlogdeltas: always return a delta info object in finddeltainfo Previously, the method returned `None` when a full snapshot was needed. The caller had to determine how to produce one itself. In practice, building a `_deltainfo` object for a full snapshot is simple. So we build it at the `finddeltainfo` level and always return a `_deltainfo` object. The caller can now simply process the `_deltainfo` return in all cases.

revlogdeltas: move special cases around raw revisions in finddeltainfo The method already contains logic for no-diff cases. Having everything in the same place is more consistent and unlocks other code improvements.