Make FakeGitRepository behave more like a real git repository (#26961)

This PR reworks the `FakeGitRepository` type that we use for testing git
interactions, to make it more realistic. In particular, the `status`
method now derives the Git status from the differences between HEAD, the
index, and the working copy. This way, if you modify a file in the
`FakeFs`, the Git repository's `status` method will reflect that
modification.

Release Notes:

- N/A

---------

Co-authored-by: Junkui Zhang <364772080@qq.com>

crates/project/src/project_tests.rs

@@ -6050,11 +6050,7 @@ async fn test_staging_hunks(cx: &mut gpui::TestAppContext) {
     )
     .await;
 
-    fs.set_head_for_repo(
-        "/dir/.git".as_ref(),
-        &[("file.txt".into(), committed_contents.clone())],
-    );
-    fs.set_index_for_repo(
+    fs.set_head_and_index_for_repo(
         "/dir/.git".as_ref(),
         &[("file.txt".into(), committed_contents.clone())],
     );