Make GitRepository::status async and remove cx parameter (#27514)

This lays the groundwork for using `status` as part of the new agent
panel.

Release Notes:

- N/A

crates/assistant2/src/thread.rs

@@ -1429,7 +1429,7 @@ impl Thread {
 
                     // Get diff asynchronously
                     let diff = repo
-                        .diff(git::repository::DiffType::HeadToWorktree, cx.clone())
+                        .diff(git::repository::DiffType::HeadToWorktree)
                         .await
                         .ok();