We've got a security report from the Phabricator team, which basically says
--config and --debugger arguments can be injected anywhere to lead to an
arbitrary command execution.

https://secure.phabricator.com/rPa7921a4448093d00defa8bd18f35b8c8f8bf3314

This is a fundamental issue of the argument parsing rules in Mercurial, which
allows extensions to populate their parsing rules and such extensions can be
loaded by "--config extensions.<name>=". There's a chicken and egg problem.
We're working on hardening the parsing rules, but which won't come in by
default as it would be a behavior change.

This patch adds a verification to reject malicious command arguments as a
last ditch. The subsequent patches will fix the problem in more appropriate
way.

Contributed by Yuya Nishihara.

git-svn-id: https://svn.redmine.org/redmine/trunk@17060 e93f8b46-1217-0410-a6f0-8f06a7374b81