Show progress indicator for /diff command (#2245)

## Summary
- Show a temporary Working on diff state in the bottom pan 
- Add `DiffResult` app event and dispatch git diff asynchronously

## Testing
- `just fmt`
- `just fix` *(fails: `let` expressions in this position are unstable)*
- `cargo test --all-features` *(fails: `let` expressions in this
position are unstable)*

------
https://chatgpt.com/codex/tasks/task_i_689a839f32b88321840a893551d5fbef

codex-rs/tui/src/bottom_pane/bottom_pane_view.rs

@@ -41,4 +41,8 @@ pub(crate) trait BottomPaneView<'a> {
     ) -> Option<ApprovalRequest> {
         Some(request)
     }
+
+    /// Optional hook for views that expose a live status line. Views that do not
+    /// support this can ignore the call.
+    fn update_status_text(&mut self, _text: String) {}
 }