Explicitly fetch kernel-mainline

Author: bmastbergen

.github/workflows/upstream-commit-check.yml

@@ -23,6 +23,10 @@ jobs:
         run: |
           git fetch origin ${{ github.base_ref }}:${{ github.base_ref }}
 
+      - name: Fetch origin/kernel-mainline
+        run: |
+          git fetch origin kernel-mainline:refs/remotes/origin/kernel-mainline
+
       - name: Ensure origin/kernel-mainline exists
         run: |
           if ! git rev-parse --verify origin/kernel-mainline >/dev/null 2>&1; then