ci: do not check --ff-only on master branch

.github/workflows/merge-check.yml

@@ -25,6 +25,9 @@ jobs:
 
       - name: Check merge --ff-only
         run: |
+          if [[ "${{ github.ref_name }}" == "master" ]]; then
+            echo "Already on master, no need to check --ff-only"
+          else
             git fetch origin master:master
             if [[ "${{ github.event_name }}" == "pull_request"* ]]; then
               git fetch origin ${{ github.event.pull_request.base.ref }}:base_branch
@@ -36,6 +39,7 @@ jobs:
               git checkout master
               git merge --ff-only ${{ github.sha }}
             fi
+          fi
 
       - name: add labels
         uses: actions-ecosystem/action-add-labels@v1