PR links for forked repo dont work in ISL

[alex-statsig]

When using sapling on a forked repo, PRs are submitted against the original repo to merge changes in. However, ISL doesnt seem to load PRs from the original repo, so it fails to show the status / PR link. The link from sl ssl works fine however.

ISL: Notice in particular the spinner next to PR number; as a result, clicking it does not open the PR

SSL:

[zzl0]

Thanks for posting the issue. cc @evangrayk @quark-zju