releasenotes: add check flag for use of admonitions and its validity
While using releasenotes extension, we will be using admonitions in commit messages.
The check (-c) flag will look for an admonition within the message. If it exists, it will
verify if it is stated under default or custom admonition. The check fails if the
admonition is not present in any of them. It also suggests similar admonitions
in case the admonition is invalid.
Differential Revision: https://phab.mercurial-scm.org/D368
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).