fix: CI: correct branch-off point for gitlint

* Changed branch-off point for gitlinting from `dev` to `origin/dev`
* Since GitLab uses the git checkout policy for the CI runners the
entire Zephyr history was checked, whereas now it is indeed only the new
feature branch which gets checked back to its branch-off point from the
dev branch.

Closes zephyrproject-rtos#26

Signed-Off-By: Alexander Preißner <alexander.preissner@blik.io>

.gitlab-ci.yml

@@ -58,7 +58,7 @@ before_script:
 # If no `.gitlint` file is present, a default set of rules is applied.
 gitlint:
   script:
-    - gitlint --commits "$(git merge-base --fork-point dev)..."
+    - gitlint --commits "$(git merge-base --fork-point origin/dev)..."
 
 # Codestyle parsing
 # This uses Zephyr's own checkpatch script to check against the diff down to the