formula_auditor: allow references to closed PRs

The `github_issue_comment` audit disallows references to closed or
merged PRs in other repositories. We should allow those, since it is a
common pattern to reference merged PRs in formulae when adding comments
that explain changes that need to be made in future versions (e.g.
`patch` blocks).

Library/Homebrew/formula_auditor.rb

@@ -829,7 +829,9 @@ module Homebrew
         next if "#{owner}/#{repo}" == formula.tap.remote_repo || owner == "Homebrew"
 
         issue = GitHub::API.open_rest("https://api.github.com/repos/#{owner}/#{repo}/issues/#{id}")
-        next if issue.blank? || issue["state"] == "open"
+        next if issue.blank?
+        next if issue["pull_request"].present?
+        next if issue["state"] == "open"
 
         issue_url = "https://github.com/#{owner}/#{repo}/#{type}/#{id}"
         problem "Formula refers to a GitHub issue or pull request that is closed: #{issue_url}"