CI: split files from revisions in git diff command (OSGeo#3775)

Co-authored-by: Edouard Choinière <echoix@users.noreply.github.com>

.github/workflows/periodic_update.yml

@@ -28,7 +28,7 @@ jobs:
           wget http://git.savannah.gnu.org/cgit/config.git/plain/config.guess && chmod +x config.guess
           wget http://git.savannah.gnu.org/cgit/config.git/plain/config.sub && chmod +x config.sub
       # Display changes, only to follow along in the logs.
-      - run: git diff config.guess config.sub
+      - run: git diff -- config.guess config.sub
       - name: Double check if files are modified
         run: git status --ignored
       - name: Create Pull Request