Make ghstack checkout produce a reusable local checkout instead of leaving users on the remote orig …

Make ghstack checkout produce a reusable local checkout instead of leaving users on the remote orig …: a task in MiMo-V2.6-RL-harbor-code: MiMo-V2.6-RL Code (Harbor) (Harbor dataset). Extend the public ghstack.checkout.main callable with optional branch: Optional str = None , force: bool = False …