ci: Output diff of uncrustify check

Not having the diff is a bit painfull when the local version of
uncrustify differs from the one on the CI runner. So uncrustify in-place
and output the diff.

.gitlab-ci.yml

@@ -103,7 +103,9 @@ test_indent:
     variables:
       - $CI_PIPELINE_SOURCE == "schedule"
   script:
-    - scripts/uncrustify.sh --check
+    - scripts/uncrustify.sh
+    - git diff
+    - "! git status -s | grep -q ."
 
 .flatpak_script_template: &flatpak_script
     script: