admin-command: add verify command
Start using the 'admin' namespace by adding a 'verify' command.
Invocation is 'admin::verify'.
The idea is to progressively add more focused checks than the existing
verify command.
To do so we need an advanced way to express what we want to check.
The first check for admin::verify is 'working-copy.dirstate' which has
no options, because it was an easy first check to implement, which
verifies the integrity of the dirstate.
This changeset was created with the help of Franck Bret.
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).