pr refs shouldn't be under heads/

[jgraham]

When we pull prs from GitHub we end up with refs unders refs/heads/pr/. This is usually fine, except if someone creates a branch with the name pr/<number> upstream, in which case we get collisions. We should instead fetch PRs to refs/pulls/<number> so there can't be a collision. This may require some changes because the resulting refs won't show up in a list of branches (which are usually taken to be things under refs/heads/ only).