GitHub App: use PR endpoint to get PR (#12816)
GH considers every pull request an issue, but not every issue is a pull
request... And when it comes to permissions, when you try fetching a PR
from a private repository using the issues endpoint, you won't be able
to use it if your app doesn't have the issues permission, but if you
want to list/post/edit a "normal" comment, you can use the issues
endpoint :_
