ci: Fetch the full history to identify the parent commit

To prevent a failure like in https://github.com/akirak/melpa/actions/runs/6979450103

.github/workflows/reset-on-merge.yml

@@ -20,6 +20,7 @@ jobs:
 
     - uses: actions/checkout@v4
       with:
+        fetch-depth: 0
         ref: ${{ github.base_ref }}
 
     - name: Tag the last head revision