diff against master (no origin), print needed formatting change

- also, don't run any clang-format if no c++ files were found

.gitlab-ci.yml

@@ -13,5 +13,4 @@ clang_format:
     - update-ca-certificates
   script:     
     - wget -c --output-document=.clang-format https://invent.kde.org/frameworks/extra-cmake-modules/raw/master/kde-modules/clang-format.cmake
-    - git diff-tree --no-commit-id --name-only -r origin/$CI_MERGE_REQUEST_TARGET_BRANCH_NAME -r $CI_COMMIT_SHA | grep -P '[.](c|cc|cpp|hpp|h)$'| xargs --verbose clang-format-11 -i
-    - git diff --exit-code
+    - git diff-tree --no-commit-id --name-only -r $CI_MERGE_REQUEST_TARGET_BRANCH_NAME -r $CI_COMMIT_SHA | grep -P '[.](c|cc|cpp|hpp|h)$' | xargs --no-run-if-empty --verbose clang-format-11 --dry-run -Werror