retain diff output if clang-format changes something

It's major PITA trying to guess what exactly clang-format has changed,
so how CI stores patch file with changes which can be applied locally if
needed.

.gitlab-ci.yml

@@ -446,7 +446,13 @@ clang-format:
   needs: []
   script:
     - if [ -r .clang-format ]; then "${CLANG_FORMAT}" -i -style=file $(git ls-files '*.c' '*.h'); fi
+    - git diff > clang-format.patch
     - if test "$(git status --porcelain | grep -Ev '\?\?' | wc -l)" -gt "0"; then git status --short; exit 1; fi
+  artifacts:
+    paths:
+      - clang-format.patch
+    expire_in: "1 week"
+    when: on_failure
 
 coccinelle:
   <<: *precheck_job