JJ's use of the Git file format is done via https://github.com/GitoxideLabs/gitoxide -- there's still some missing features in the networking part, but the local-file-manipulation features are fully there.
Great lib. I meant libdog, a different thing. gitoxide is like "git implemented in Rust" while libdog is "git's necessary primitives in C" (objects, packs, sync proto, own index format, diff/merge, etc). As a result, gitoxide is 270KLoC while libdog is 24KLoC, plus 44KLoC of tokenizers. libdog uses token-level algorithms for diff, merge, blame. It also does syntax highlighting in the pager - they all need grammars.