Fixing the push case (#132)

Quick fix

I'm assuming we're operating under the LLVM developer policies which allow no-review for obvious fixes...

.github/workflows/buildAndTest.yml

@@ -91,8 +91,9 @@ jobs:
         fi
         echo "DIFF_COMMIT=$DIFF_COMMIT" >> $GITHUB_ENV
     - name: git fetch base commit
+      continue-on-error: true
       run: |
-        git fetch origin $DIFF_COMMIT
+        [[ ! "$DIFF_COMMIT" == *"HEAD"* ]] && git fetch origin $DIFF_COMMIT
     - name: clang-format
       if: ${{ always() }}
       run: |