I have never used this tool but Google open-sourced Copybara[1]. If you look at the commits of their other open-source repositories, it seems this tool is often used.
Disclosure: I work at Google, but not on Copybara.
I've seen Copybara used at Google in both directions: for some projects, the internal repo is the authoritative one, and for others, the external repo is authoritative.
Copybara is not prescriptive, you can go in either direction.
Looks a bit heavy though for my liking but maybe they have some good ideas.
> Copybara requires you to choose one of the repositories to be the authoritative repository
Looks like it is more angled at mirroring private to public.