cc @SomeoneToIgnore

## Summary

Follow-up to #56155. I extracted the remaining git related things (again
not all of them, leftovers are more tricky as there are git + fold +
some others things combined) into `git.rs`

We nod reached a good milestone, the `editor.rs` would be below 20k
lines now

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- N/A