The Hg plugin is useful if you would like to track changes to your Bcfg2 repository using Hg backend. Currently, it enables you to get revision information out of your repository for reporting purposes.
You will need to install Mercurial on the Bcfg2 server first.
Simply add Hg to your plugins line in /etc/bcfg2.conf:
[server] plugins = Base,Bundler,Cfg,...,Hg