Show diff and hardcode linght

`-l, --line-length INTEGER       How many characters per line to allow.
                                  [default: 88]
`

.gitlab-ci.yml

@@ -66,7 +66,7 @@ black:
     - >
       if [ ! -z "$FLIST" ];
       then
-        black --check $FLIST;
+        black --check --diff $FLIST;
       else
         echo "No files to lint"
         true;