Rails db:schema:dump after db:migrate:redo patch

I've submitted another patch to Rails for a minor-but-annoying bug – if you use db:migrate:redo you'll find that it does migrate correctly but the schema dump afterwards doesn't work, so db:test:prepare clones the wrong structure (for example).

The patch fixes things so that second and subsequent db:schema:dump invocations work properly. Testing and +1s, as always, very welcome.