Narrow refresh populate_existing to just refresh_state
Fixed additional regression caused by the "eagerloaders on refresh" feature
added in :ticket:`1763` where the refresh operation historically would set
``populate_existing``, which given the new feature now overwrites pending
changes on eagerly loaded objects when autoflush is false. The
populate_existing flag has been turned off for this case and a more
specific method used to ensure the correct attributes refreshed.