Update the focus handle returned by the `GitPanel::focus_handle` method
in order to ensure that, if the commit editor is currently expanded,
using the `git_panel::ToggleFocus` action will correctly focus back on
the expanded editor, rather than on the panel.

# Objective

When using the expanded commit editor, if one focuses away from the
panel and then uses the `git panel: toggle focus` action in order to
focus back on the panel, the focus will land on the panel, which is now
being rendered behind the commit editor.

## Solution

Update the focus handle returned by the `GitPanel` to take into
consideration whether the commit editor is expanded and, in that case,
return the editor's focus handle instead.

## Testing

Tested both manually as well as through a newly introduced test –
`git_ui::git_panel::tests::test_focus_handle`.

## Self-Review Checklist:

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

## Showcase

<details>
  <summary>Before</summary>


https://github.com/user-attachments/assets/0e2647f3-2835-485f-8f01-2128d2de6b8e
</details>

<details>
  <summary>After</summary>


https://github.com/user-attachments/assets/cc77fae7-e76c-4389-82b0-f4e7ec8258d7
</details>

---

Release Notes:

- Fixed `git panel: toggle focus` to focus back on the commit editor in
case it is expanded.