git: Enable git stash in git panel (#32821)

Related discussion #31484

Release Notes:

- Added a menu entry on the git panel to git stash and git pop stash. 

Preview: 


![Screenshot-2025-06-17_08:26:36](https://github.com/user-attachments/assets/d3699ba4-511f-4c7b-a7cc-00a295d01f64)

---------

Co-authored-by: Cole Miller <cole@zed.dev>

crates/fs/src/fake_git_repo.rs

@@ -398,6 +398,18 @@ impl GitRepository for FakeGitRepository {
         })
     }
 
+    fn stash_paths(
+        &self,
+        _paths: Vec<RepoPath>,
+        _env: Arc<HashMap<String, String>>,
+    ) -> BoxFuture<Result<()>> {
+        unimplemented!()
+    }
+
+    fn stash_pop(&self, _env: Arc<HashMap<String, String>>) -> BoxFuture<Result<()>> {
+        unimplemented!()
+    }
+
     fn commit(
         &self,
         _message: gpui::SharedString,