..by explicitly fetching refs/pulls/:idx/head from the base repo.
Sorry, I mixed this with a split-up of PullCheckout(). I can try to separate that, if preferred
split up that big function, not sure if it helps with readability
change in behaviour: now also allows checking out a PR whose head branch
was deleted already.
No due date set.
Deleting a branch is permanent. It CANNOT be undone. Continue?