Do not include sub-entities when fetching unreferenced objects.

This is to fix a bug where an object belonging to a sub-entity would be fetched *and* matched by the predicate since it is logically not in the object IDs set. The object would thus be deleted being seen as unreferenced.
Merge pull request #119 from realmacsoftware/subentities-deletion

Do not include sub-entities when fetching unreferenced objects.