fix before_models_committed to actually fire with stuff

As highlighted by @insom before_models_committed will not fire because it has no changes.

Also see https://github.com/mitsuhiko/flask-sqlalchemy/pull/117
and https://github.com/mitsuhiko/flask-sqlalchemy/pull/95

Its plainly obvious this is broken (even the simplest of test cases shows it) When will any of these fixed be pulled into main???