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