GitHub is not open source.

They use an api called libgit to interface with all the repos that you can find here https://github.com/libgit2/libgit2

If they dogfood, then it would be in a private repo on github. You could get a job there or, for the impatient, just wait until the next RoR exploit.

GitLab is an open-source alternative: http://gitlab.org/

I have tried GitLab and its stable to an extent - but not as feature rich as GitHub. But, still for smaller teams that want a hosted solution, I can second to use it.