Allow ghstack to checkout prs created without ghstack.

[eellison]

This would be nice to have in the case where someone is not using ghstack so it works with my existing workflow.

[ezyang]

sure, patches accepted

[ezyang]

btw, I use gh pr checkout URL from the official gh tool to do this. Although it annoyingly insists on making a branch and fails you if the branch exists and is stale