ci/github-script/labels: close empty PRs

If the change of a PR has already been merged to the target branch
elsewhere, the PR will not be auto-closed by GitHub - and will still
show the same original diff. Still, the temporary merge commit is
actually empty. This causes all kinds of strange CI behavior, from not
showing rebuilds to not pinging maintainers.

We check the merge commit during labeling anyway, to see whether a merge
conflict is present. It's easy to just look a the number of affected
files in this merge commit - and if there are none, we can just
automatically close the PR as no longer relevant.

ci/github-script/labels.js

@@ -20,6 +20,46 @@ module.exports = async ({ github, context, core, dry }) => {
       })
     ).data
 
+    // When the same change has already been merged to the target branch, a PR will still be
+    // open and display the same changes - but will not actually have any effect. This causes
+    // strange CI behavior, because the diff of the merge-commit is empty, no rebuilds will
+    // be detected, no maintainers pinged.
+    // We can just check the temporary merge commit, and if it's empty the PR can safely be
+    // closed - there are no further changes.
+    if (pull_request.merge_commit_sha) {
+      const commit = (
+        await github.rest.repos.getCommit({
+          ...context.repo,
+          ref: pull_request.merge_commit_sha,
+        })
+      ).data
+
+      if (commit.files.length === 0) {
+        const body = [
+          `The diff for the temporary merge commit ${pull_request.merge_commit_sha} is empty.`,
+          'The changes in this PR have almost certainly already been merged to the target branch.',
+        ].join('\n')
+
+        core.info(`PR #${item.number}: closed`)
+
+        if (!dry) {
+          await github.rest.issues.createComment({
+            ...context.repo,
+            issue_number: pull_number,
+            body,
+          })
+
+          await github.rest.pulls.update({
+            ...context.repo,
+            pull_number,
+            state: 'closed',
+          })
+        }
+
+        return {}
+      }
+    }
+
     const reviews = await github.paginate(github.rest.pulls.listReviews, {
       ...context.repo,
       pull_number,