Hg-Git Mercurial Plugin: "This is the Hg-Git plugin for Mercurial, adding the ability to push to and pull from a Git server repository from Mercurial. This means you can collaborate on Git based projects from Mercurial, or use a Git server as a collaboration point for a team with developers using both Git and Mercurial."
If using TortoiseHg, the instructions on enabling the plugin are here. As dulwich is already installed with TortoiseHg, all you need to do is clone the hg-git repository and enable the extension in your mercurial.ini.
An important feature this plugin brings is the sharing of bookmarks, similar to Git branches.
"It is intended that Hg users may wish to use this to collaborate even if no Git users are involved in the project, as it may provide some advantages if you're using Bookmarks (it will transfer bookmarks on push, which Mercurial doesn't do)."