Investigate gitoxide

[Xophmeister]

Topiary currently shells out to Git when fetching grammars dynamically. Originally, IIRC, we used a crate that wraps libgit2; however we couldn’t get this to build in the macOS CI (hence the change).

Gitoxide is a pure-Rust reimplementation of Git and looks mature enough to do everything we need.

Removing the need to shell out, without weird dependency problems, seems like a win.