Fork tree prune assumptions removal v2 (#13327)

* Removed assumptions about ancestry from fork tree prune method

* Tests improvement

* Fork tree prune refactory

* Code refactory

* Correctly handle borderline, but legit, case

* Apply suggestions from code review

Co-authored-by: André Silva <[email protected]>

* Removed duplicated test

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Co-authored-by: André Silva <[email protected]>