Extern: merging fails for is_referenced_by if both entities are created on the fly

extern: https://gitlab.com/linkahead/linkahead-crawler/-/issues/95

Merging two entities that are referenced by a third entity fails with this identifiable:

Experiment:
- date
Event:
- is_referenced_by: Experiment
- event_id

• Is fixed.
• Followup issues created if necessary