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/collab/src/tests/git_tests.rs

@@ -103,7 +103,6 @@ async fn test_project_diff(cx_a: &mut TestAppContext, cx_b: &mut TestAppContext)
             }),
         )
         .await;
-    client_a.fs().recalculate_git_status(Path::new("/a/.git"));
     cx_b.run_until_parked();
 
     project_b.update(cx_b, |project, cx| {