Support jujutsu (jj) VCS

[dbarnett]

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Would it be possible to support https://github.com/martinvonz/jj so that if I clone a repo of a python project using that, I'm still able to build its python package contents normally?

At least in the case of their git backend that could be as simple as detecting if the repo dir contains a .jj/repo/store/git/ and running the existing git detection against that $GIT_DIR (example: https://martinvonz.github.io/jj/latest/github/#using-github-cli).