Fix add_all_untracked to return code as expected

GitAddAllUnstagedHandler in handlers.py expects the dictionary returned by this function to have "code" as a key. Seems like this function is the only one that had it's return statement not modified by: 80aee60#diff-041cb4cfb7143f3bbcdd07d28dffa92c

Author: ianhi

jupyterlab_git/git.py

@@ -592,7 +592,7 @@ async def add_all_unstaged(self, top_repo_path):
 
         if code != 0:
             return {"code": code, "command": " ".join(cmd), "message": error}
-        return {"result": code}
+        return {"code": code}
 
     async def add_all_untracked(self, top_repo_path):
         """