git: Don't consider $HOME as containing git repository unless it's opened directly (#25948)

When a worktree is created, we walk up the ancestors of the root path
trying to find a git repository. In particular, if your `$HOME` is a git
repository and you open some subdirectory of `$HOME` that's *not* a git
repository, we end up scanning `$HOME` and everything under it looking
for changed and untracked files, which is often pretty slow. Consistency
here is not very useful and leads to a bad experience.

This PR adds a special case to not consider `$HOME` as a containing git
repository, unless you ask for it by doing the equivalent of `zed ~`.

Release Notes:

- Changed the behavior of git features to not treat `$HOME` as a git
repository unless opened directly

crates/fs/src/fs.rs

@@ -135,6 +135,7 @@ pub trait Fs: Send + Sync {
         Arc<dyn Watcher>,
     );
 
+    fn home_dir(&self) -> Option<PathBuf>;
     fn open_repo(&self, abs_dot_git: &Path) -> Option<Arc<dyn GitRepository>>;
     fn is_fake(&self) -> bool;
     async fn is_case_sensitive(&self) -> Result<bool>;
@@ -813,6 +814,10 @@ impl Fs for RealFs {
         temp_dir.close()?;
         case_sensitive
     }
+
+    fn home_dir(&self) -> Option<PathBuf> {
+        Some(paths::home_dir().clone())
+    }
 }
 
 #[cfg(not(any(target_os = "linux", target_os = "freebsd")))]
@@ -846,6 +851,7 @@ struct FakeFsState {
     metadata_call_count: usize,
     read_dir_call_count: usize,
     moves: std::collections::HashMap<u64, PathBuf>,
+    home_dir: Option<PathBuf>,
 }
 
 #[cfg(any(test, feature = "test-support"))]
@@ -1031,6 +1037,7 @@ impl FakeFs {
                 read_dir_call_count: 0,
                 metadata_call_count: 0,
                 moves: Default::default(),
+                home_dir: None,
             }),
         });
 
@@ -1524,6 +1531,10 @@ impl FakeFs {
     fn simulate_random_delay(&self) -> impl futures::Future<Output = ()> {
         self.executor.simulate_random_delay()
     }
+
+    pub fn set_home_dir(&self, home_dir: PathBuf) {
+        self.state.lock().home_dir = Some(home_dir);
+    }
 }
 
 #[cfg(any(test, feature = "test-support"))]
@@ -2079,6 +2090,10 @@ impl Fs for FakeFs {
     fn as_fake(&self) -> Arc<FakeFs> {
         self.this.upgrade().unwrap()
     }
+
+    fn home_dir(&self) -> Option<PathBuf> {
+        self.state.lock().home_dir.clone()
+    }
 }
 
 fn chunks(rope: &Rope, line_ending: LineEnding) -> impl Iterator<Item = &str> {