We just merged our first pull requests on github! We wanted to wait on doing this until the mirror bot was updated, which is now complete. If you have a commit bit, you should feel free to merge PRs that have undergone sufficient review. You can also indicate that you reviewed and approved a PR by either commenting on it or by going through the github code review interface (see https://help.github.com/articles/about-pull-request-reviews/ ).
Thanks very much to everyone who helped out with this effort! So far I think it's been pretty painless (knocks on wood). That said, please let us know if you run into any issues (e.g. with yt setups that use a source install of yt from a mercurial repo).