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/src/git.rs

@@ -5,12 +5,6 @@ mod remote;
 pub mod repository;
 pub mod status;
 
-#[cfg(any(test, feature = "test-support"))]
-mod fake_repository;
-
-#[cfg(any(test, feature = "test-support"))]
-pub use fake_repository::*;
-
 pub use crate::hosting_provider::*;
 pub use crate::remote::*;
 use anyhow::{anyhow, Context as _, Result};