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/app_event.rs

@@ -51,6 +51,9 @@ pub(crate) enum AppEvent {
         matches: Vec<FileMatch>,
     },
 
+    /// Result of computing a `/diff` command.
+    DiffResult(String),
+
     InsertHistory(Vec<Line<'static>>),
 
     StartCommitAnimation,