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/repository.rs

@@ -1,7 +1,6 @@
 use crate::status::GitStatus;
 use crate::SHORT_SHA_LENGTH;
 use anyhow::{anyhow, Context as _, Result};
-use askpass::{AskPassResult, AskPassSession};
 use collections::HashMap;
 use futures::future::BoxFuture;
 use futures::{select_biased, AsyncWriteExt, FutureExt as _};
@@ -24,6 +23,8 @@ use sum_tree::MapSeekTarget;
 use util::command::new_smol_command;
 use util::ResultExt;
 
+pub use askpass::{AskPassResult, AskPassSession};
+
 pub const REMOTE_CANCELLED_BY_USER: &str = "Operation cancelled by user";
 
 #[derive(Clone, Debug, Hash, PartialEq, Eq)]
@@ -311,11 +312,13 @@ pub struct RealGitRepository {
 }
 
 impl RealGitRepository {
-    pub fn new(repository: git2::Repository, git_binary_path: Option<PathBuf>) -> Self {
-        Self {
+    pub fn new(dotgit_path: &Path, git_binary_path: Option<PathBuf>) -> Option<Self> {
+        let workdir_root = dotgit_path.parent()?;
+        let repository = git2::Repository::open(workdir_root).log_err()?;
+        Some(Self {
             repository: Arc::new(Mutex::new(repository)),
             git_binary_path: git_binary_path.unwrap_or_else(|| PathBuf::from("git")),
-        }
+        })
     }
 
     fn working_directory(&self) -> Result<PathBuf> {