revlogdeltas: pass revision number to _candidatesgroups The `_candidates_groups` logic works on revisions, so the nodes are currently just converted to revs at the top of the function. It seems cleaner to instead pass revision numbers to the function. Having this conversion out of the way will help for further cleanups.

revlogdeltas: extract _getcandidaterevs in a function The logic barely uses the object it is attached to. This is an important function that we will clean up in the coming changesets. Moving it at the top level helps us with that cleanup.

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.