[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.

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