Closes #31155

Release Notes:

- Ensure `git_panel::FocusChanges` bypasses the panel’s `Focusable`
logic and directly focuses the `ChangesList` handle so the command works
even when the repository has no entries.
- Keep the `Focusable` behavior from the commit 45b126a (which routes
empty panels to the commit editor) by handling this special-case action
rather than regressing the default focus experience.