The ORM didn't reuse joins for direct foreign key traversals when using
chained filters. For example:
    qs.filter(fk__somefield=1).filter(fk__somefield=2))
produced two joins.

As a bonus, reverse onetoone filters can now reuse joins correctly

The regression was caused by the join() method refactor in commit
68847135

Thanks for Simon Charette for spotting some issues with the first draft
of the patch.