Restore format patch, --exit-code unkown option

{{cookiecutter.project_slug}}/.gitlab-ci.yml

@@ -40,7 +40,10 @@ format:
   script:
     - scripts/format-cpp.sh
     - scripts/format-cmake.sh
-    - git diff --exit-code 1
+    - git diff > format.patch
+      # Fail when the patch is not empy (note ! can't be used directly.)
+    - (! grep -q '^--- a'  format.patch)
+
   artifacts:
     when: on_failure
     paths: