Fetching pull requests can take a long time, and we don't want to delay
the refresh by it; in particular, for a WithWaitingStatusBlockingInput
we want the UI thread to be unblocked again while pull requests are
still fetching in the background. This is similar to how we fetch the
behind values for branches in BranchLoader; this will update the UI
without much flicker when done, and doesn't have to block anything.