`github.import_labels_as_tags` doesn't work for pull requests

Looking at the REST responses from GitHub for pull requests, I don't see a 'labels' field (or any field containing my labels, for that matter).  Issues, on the other hand, include a label.

For example, compare
 * https://api.github.com/repos/buildbot/buildbot/issues/1380
 * https://api.github.com/repos/buildbot/buildbot/pull/1380

I think there are two things to fix.  First, even repos with a false 'has_issues' should have their issues downloaded; and second, rather than discarding issues for which there's a pull request, merge them.

But now that I think about it, why request pull reqs at all?  Why not just get all of the issues (since they include pull reqs, too)?  You can identify issues that are pull requests because they have a 'pull_request' key.

Given an answer to the latter question, I can try to implement this.