git: Add support for opening git worktrees (#20164)

This adds support for [git
worktrees](https://matklad.github.io/2024/07/25/git-worktrees.html). It
fixes the errors that show up (git blame not working) and actually adds
support for detecting git changes in a `.git` folder that's outside of
our path (and not even in the ancestor chain of our root path).

(While working on this we discovered that our `.gitignore` handling is
not 100% correct. For example: we do stop processing `.gitignore` files
once we found a `.git` repository and don't go further up the ancestors,
which is correct, but then we also don't take into account the
`excludesFile` that a user might have configured, see:
https://git-scm.com/docs/gitignore)


Closes https://github.com/zed-industries/zed/issues/19842
Closes https://github.com/zed-industries/zed/issues/4670

Release Notes:

- Added support for git worktrees. Zed can now open git worktrees and
the git status in them is correctly handled.

---------

Co-authored-by: Antonio <antonio@zed.dev>
Co-authored-by: Bennet <bennet@zed.dev>

crates/git/src/repository.rs

@@ -45,6 +45,8 @@ pub trait GitRepository: Send + Sync {
     fn branch_exits(&self, _: &str) -> Result<bool>;
 
     fn blame(&self, path: &Path, content: Rope) -> Result<crate::blame::Blame>;
+
+    fn path(&self) -> PathBuf;
 }
 
 impl std::fmt::Debug for dyn GitRepository {
@@ -83,6 +85,11 @@ impl GitRepository for RealGitRepository {
         }
     }
 
+    fn path(&self) -> PathBuf {
+        let repo = self.repository.lock();
+        repo.path().into()
+    }
+
     fn load_index_text(&self, relative_file_path: &Path) -> Option<String> {
         fn logic(repo: &git2::Repository, relative_file_path: &Path) -> Result<Option<String>> {
             const STAGE_NORMAL: i32 = 0;
@@ -276,6 +283,11 @@ impl GitRepository for FakeGitRepository {
         None
     }
 
+    fn path(&self) -> PathBuf {
+        let state = self.state.lock();
+        state.path.clone()
+    }
+
     fn status(&self, path_prefixes: &[PathBuf]) -> Result<GitStatus> {
         let state = self.state.lock();
         let mut entries = state