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.

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.

revlogdeltas: extra fulltext building in its own function The process of building a full text from a delta is complex enough to deserve isolation. The previous function mixed cache management and computation, we now have clearer boundaries and simpler functions/methods.

revlog: split functionality related to deltas computation in a new module The revlog module is getting big and this logic is getting more and more advanced. Moving it to `mercurial.revlogutils.deltas` split a lot off revlog.py and will help this logic to become less interleaved with revlog. The code is simply moved without modification (but for namespace changes). Refactoring and improvement will be made in later changesets.

revlog: split constants into a new `revlogutils.constants` module We want to split some logic out of the main revlog file (the delta computing logic). However, this logic needs access to multiple constants related to the revlog. So we move all revlog related constants into a new module that could be imported from multiple places. We don't copy the file (preserving blame history) because there are only a few moving lines. Also, copying the file would result in annoying merge conflicts with ongoing work from others contributors.

rebase: skip extinct revisions even if it has no successor in rebase set Differential Revision: https://phab.mercurial-scm.org/D4408

tests: add test showing that rebase of extinct commit with successor fails As the test case shows, attempting to rebase a commit that has a successor that is not in the rebase set and not in the destination currently fails because it "will cause divergences". However, it doesn't seem like there's any harm in skipping the extinct commit. I suspect this case missed simply because extinct revisions are usually hidden. Differential Revision: https://phab.mercurial-scm.org/D4407