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)
- [ ] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Summary:

- Added split/unified diff controls to the commit history commit view
using `SplittableEditor`.
- Kept commit messages separate from file diffs with an 8-line
scrollable message panel.
- Fixed split setup edge cases and async/test type mismatches from the
commit view changes.

Verification:

- `cargo check -p git_ui`
- `cargo check -p git_ui --tests`
- `cargo check -p editor --tests`
- `./script/clippy -p git_ui`
- `git diff --check`

Release Notes:

- Improved the commit history diff view by adding the ability to toggle
the split diff mode.

---------

Co-authored-by: Danilo Leal <daniloleal09@gmail.com>