fix: CI: create local diff in relation to origin/dev

* The dev branch may not be available locally, so that `git diff` could fail.
* Creating the patch file in relation to origin/dev always works

Closes zephyrproject-rtos#22

Author: Alexander Preißner

.gitlab-ci.yml

@@ -62,5 +62,5 @@ gitlint:
 # warnings.
 codestyle:
   script:
-    - git diff dev > branch.patch
+    - git diff origin/dev > branch.patch
     - ./scripts/checkpatch.pl branch.patch