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

Run Mercurial tests with Vagrant:

$ vagrant up
$ vagrant ssh -c ./run-tests.sh