So, I think there's a couple relevant issues when talking about decentralizing a github like service. There a couple of things Github provides, and my naive approach to making GitHub less essential would be to address them one by one:

* Issue tracking. It's a bit hard to export these. I imagine it'd be a bit nicer to export if issues were actually stored inside git as trees. You could easily make the issues frontend use git as the storage backend. If users don't have an account, maybe they could issue a pull-request to the issue tracker?

* Notifications. This ones's a bit harder. Web-hooks?

* Merge requests - github uses refs internally (I think). That wouldn't be too hard to standardize. If somebody adds commits to their pull/merge request, then they just have to push the updated ref to the repo they're submitting to.

* Auth - this is hardest part. GitHub provides authentication & permissions for pushing. The issue with allowing merge requests submission from anybody (which you could do with a server side hook) is that now people can ddos you by submitting HUGE pull requests constantly. If you could make that safe, in theory you could get away without needing user account federation.

* There is also the big security issue that you can't actually use server side hooks for this stuff since libgit2 can be used to bypass it (when pushing using libgit2, it doesn't trigger server side hooks).

* Oh, and this entire time I've been thinking about the email & username as being unspoofable. But you can easily spoof them. I guess you need federated user accounts after all.

Edit: It's untrusted interactions that's hard to decentralize ---------------

All in all, I think that git is easily decentralized if you know you can trust all the actors. It's untrusted git that's harder to decentralize.