revlogdeltas: split candidate groups selection from the filtering logic The group iteration has two main components: * walking candidates, the logic that we are about to extend to build intermediate snapshots, * Making sure we test diffs against interesting bases. No duplicated tests, skipping empty revisions, etc. We split `_candidategroups` to separate the two components. This achieves two goals: * It will be simpler to update the walking logic for intermediate snapshots, * We can gather the filtering logic from `finddeltainfo` into `_candidategroups` to centralize it.

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.