This is helpful when the pull request has been fixed up and you already have a local branch.
Otherwise, we can end up fetching a repo twice.
This allows it to work in bare repositories too.
While we're at it, use the git_pager too.
Unfortunately, the code is duplicated (because I want git-missing to be standalone). Use the function for git-missing and the prompt.
--get will make sure we get an error code if it doesn't exist.