obsolete: drop usage of changectx in '_computephasedivergentset'
Changectx are expensive and not needed there. The use of `repo.set` denote old
code that predate the introduction of `repo.revs` that we now use.
On my mercurial repository 495 draft:
before: 0.010275 second
after: 0.008832 second
On a mercurial repository with 115973 draft:
before: 0.899255 second
after: 0.397131 second
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).