Use current branch instead of hardcoded master for push

Replace hardcoded origin/master reference with @{u} (upstream of current
branch) for the diff check, and use HEAD for the push target so the
workflow works correctly on any branch.

.github/workflows/update_playlist.yml

@@ -18,4 +18,4 @@ jobs:
           python3 ./make_playlist.py
           git add .
           git diff --staged --quiet || git commit --quiet -m "Update Playlist (GitHub Actions)"
-          git diff --quiet HEAD origin/master || git push -f origin master
+          git diff --quiet HEAD @{u} || git push -f origin HEAD