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/worktree/src/worktree.rs

@@ -4292,7 +4292,11 @@ impl BackgroundScanner {
         let mut containing_git_repository = None;
         for (index, ancestor) in root_abs_path.as_path().ancestors().enumerate() {
             if index != 0 {
-                if let Ok(ignore) =
+                if Some(ancestor) == self.fs.home_dir().as_deref() {
+                    // Unless $HOME is itself the worktree root, don't consider it as a
+                    // containing git repository---expensive and likely unwanted.
+                    break;
+                } else if let Ok(ignore) =
                     build_gitignore(&ancestor.join(*GITIGNORE), self.fs.as_ref()).await
                 {
                     self.state