[rset] fix infinite recursion introduced in c1eb5a676c80
The mechanism to avoid loops in c1eb5a676c80 "[rset] Always complete
attribute/relation caches in ResultSet.get_entity" breaks down: we would
loop forever if two entities that were already in the cache were linked
by a relation with ? or 1 cardinality in both directions. To avoid
that, keep a set of already-considered columns. Related to #9942503.