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.
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).