obsolete: add a function to compute "exclusive-markers" for a set of nodes
This set will be used to select the obsmarkers to be stripped alongside the
stripped changesets. See the function docstring for details.
More advanced testing is introduced in the next changesets to keep this one
simpler. That extra testing provides more example.
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).