check-concurrency: expose the feature as 'concurrent-push-mode'
We move the feature to a proper configuration and document it. The config goes
in the 'server' section because it feels like something the server owner would
want to decide. We pick and open field because it seems likely that other
checking levels will emerge in the future. (eg: server like the mozilla-try
server will likely wants a "none" value)
The option name contains 'push' since this affects 'push' only. The option value
'check-related' is preferred over one explicitly containing 'allow' or 'deny'
because the client still have a strong decision power here. Here, the server is
just advising the client on the check mode to use.
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).