fetch base ref

Uses example in as of yet unmerged PR actions/checkout#213

Author: egpbos

.github/workflows/code_analysis.yml

@@ -15,5 +15,7 @@ jobs:
       BASE_COMMIT: ${{ github.event.pull_request.base.sha }}
     steps:
     - uses: actions/checkout@v2
+    - name: Fetch base_ref HEAD
+      run: git fetch --depth=1 origin +refs/heads/${{github.base_ref}}:refs/remotes/origin/${{github.base_ref}}
     - name: run clang-format script
       run: .ci/format_script.sh