github actions: Verify PR branch isn't on a stale base

Check that the PR branch is up to date with the latest base branch commit,
and fail if it isn't.

Author: kerneltoast

.github/workflows/validate-kernel-commits.yml

@@ -25,6 +25,13 @@ jobs:
           git fetch "${{ github.event.pull_request.head.repo.clone_url }}" "${{ github.head_ref }}"
           echo "HEAD_SHA=$(git rev-parse FETCH_HEAD)" >> "$GITHUB_ENV"
 
+      - name: Verify PR branch isn't on stale base
+        run: |
+          if ! git merge-base --is-ancestor "${{ github.base_ref }}" "$HEAD_SHA"; then
+            echo "❌ PR branch must be rebased onto latest base branch commit"
+            exit 1
+          fi
+
       - name: Checkout kernel-src-tree-tools
         uses: actions/checkout@v4
         with: