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

Addresses https://github.com/zed-industries/zed/discussions/33773.

This changes git panel file activation so double-clicking or
secondary-opening a changed file opens a dedicated full-file diff tab
backed by a `SplittableEditor`.

The per-file diff reuses the project diff staging and restore controls,
respects the configured diff view style, and focuses an existing
per-file diff tab when one is already open instead of creating
duplicates.

Verified with `cargo run`.

Release Notes:

- Improved git panel file diff opening.

---------

Co-authored-by: Christopher Biscardi <chris@christopherbiscardi.com>