annotate CONTRIBUTING @ 45144:c93dd9d9f1e6

discovery: change users of `outgoing.missingheads` to `outgoing.ancestorsof`

The attribute `missingheads` was recently renamed to `ancestorsof`, as it,
despite the old name, doesn’t contain the missing heads but the changesets that
were requested (including ancestors) for the outgoing operation.

Changing all the users enables to print a warning if the old name is used.

There is a good chance that some of the users are buggy because of the old name.
Changing them to use the new name makes it more obvious that they are buggy. All
users need to be reviewed for bugs. When sending patches for fixing them, the
change will be more obvious without having to explain again and again the
discrepancy of the old attribute name and what it actually contained.

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).