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/git_ui/src/git_panel.rs

@@ -4474,7 +4474,7 @@ mod tests {
         )
         .await;
 
-        fs.set_status_for_repo_via_git_operation(
+        fs.set_status_for_repo(
             Path::new(path!("/root/zed/.git")),
             &[
                 (