Use gh CLI to get the PR diff

This enables the workflow to get the changed file list when run on
`pull_request_target` event

.github/workflows/changelogs.yml

@@ -75,8 +75,9 @@ jobs:
       run: |
         pip install python-bugzilla~=3.2.0
 
-        CHANGED_FILES=$(git diff --name-only \
-            ${{ github.event.pull_request.base.sha }}..${{ github.event.pull_request.head.sha }})
+        CHANGED_FILES=$(gh pr diff -R ${{ github.event.pull_request.base.repo.full_name }} \
+            ${{ github.event.pull_request.number }} --name-only)
+        echo $CHANGED_FILES
 
         # Tracker file can be retrieved from the OBS/IBS API
         # by calling 'osc api /issue_trackers/' (with authentication)