Fix for r56885, r58364, r58364: use a two-part GROUP BY even on MySQL. The implicit...

Fix for r56885, r58364, r58364: use a two-part GROUP BY even on MySQL. The implicit sort shouldn't cause any problems here. Query runs without errors on MySQL (with or without ONLY_FULL_GROUP_BY), SQLite and PostgreSQL. The previous version in the non-MySQL case was plainly broken, see CR r58915.