Personally I don't think adding a layer on top of git is a good idea.
I know it's hard to break git's hegemony but it would be much more powerful and less constrained to build on top of a new foundation.
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At Google, JJ is natively supported for interfacing with the monorepo, this is where the proof of being backend agnostic comes from. Hopefully as the network effects catches on we'll see more and more of a desire for 3P forges.