Jujutsu is not a Google product. Jujutsu is not a fork of anything, so upstreaming it does not make sense. It does support git repos, but that is a mandatory feature for new version control software to compete with git as people don't want a VCS that is not compatible with one's existing repos. It doesn't mean it's a fork of git.
It's a downstream consumer of (lib)git.
It looks to me like they just use libgit as a dependency. Do they have a fork of libgit that they would push changes upstream from?
You seem really fixated on this "fork" thing, which is not mentioned anywhere in my comments.