Fix git stage race condition with delayed fs events (#27036)

This PR adds a failing test `test_staging_hunks_with_delayed_fs_event`
and makes it pass

Also skips a queued read for git diff states if another read was
requested (less work)

This still doesn't catch all race conditions, but the PR is getting long
so I'll yield this and start another branch

Release Notes:

- N/A

crates/git/src/fake_repository.rs

@@ -58,16 +58,26 @@ impl FakeGitRepositoryState {
 impl GitRepository for FakeGitRepository {
     fn reload_index(&self) {}
 
-    fn load_index_text(&self, path: RepoPath, _: AsyncApp) -> BoxFuture<Option<String>> {
+    fn load_index_text(&self, path: RepoPath, cx: AsyncApp) -> BoxFuture<Option<String>> {
         let state = self.state.lock();
         let content = state.index_contents.get(path.as_ref()).cloned();
-        async { content }.boxed()
+        let executor = cx.background_executor().clone();
+        async move {
+            executor.simulate_random_delay().await;
+            content
+        }
+        .boxed()
     }
 
-    fn load_committed_text(&self, path: RepoPath, _: AsyncApp) -> BoxFuture<Option<String>> {
+    fn load_committed_text(&self, path: RepoPath, cx: AsyncApp) -> BoxFuture<Option<String>> {
         let state = self.state.lock();
         let content = state.head_contents.get(path.as_ref()).cloned();
-        async { content }.boxed()
+        let executor = cx.background_executor().clone();
+        async move {
+            executor.simulate_random_delay().await;
+            content
+        }
+        .boxed()
     }
 
     fn set_index_text(