I think we also have to be honest about what the project here is too, it's not to have both C and Rust together, but to replace all C with Rust. In which case, it probably makes sense to just clone to repo and work on a fork like they did with SSH.
Some git subcommands are implemented in these. git filter-branch is a shell script, git cvsimport is a Perl script, and git p4 (perforce interop) is a Python script. There are not too many left these days (git add -p/-i also used to call a Perl script), but they exist.
[1] https://www.europarl.europa.eu/legislative-train/
[2] https://www.europarl.europa.eu/legislative-train/theme-a-new...