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That's a great thought. For my own edification, which third party apps come to mind?

At present, the "official" collaboration tools in the GitHub integrations directory are here: https://github.com/integrations/feature/collaborate

I don't understand it :)

GitHub has become the de-facto place to host open source projects. Improving the tooling around code merging, seems like a no-brainer, and a huge potential win for the projects using it, especially the larger ones.

I agree. I think there's a whole ecosystem there.