Finish replacing `git2` with `git-repository`

[dbanty]

I've been slowly replacing usages of git2 with git-repository and related crates from gitoxide. These crates have friendlier interfaces and documentation since they aren't just bindings to C libraries.

It would be great to finish this transition, so we aren't carrying around two separate Git implementations 😅.

[dbanty]

Waiting on GitoxideLabs/gitoxide#301

[dbanty]

Ended up reverting entirely to git2 instead so we don't need both. We can revisit the gix stuff if/when it's got all the features we need.