Referring the same entity multiple times breaks _referringItem

Problem/Motivation

EntityReferenceFormatterBase::getEntitiesToView() sets $entity->_referringItem but if the same entity is referred multiple times then the value gets overridden.

Steps to reproduce

Refer the same file twice, add something to $item->_attributes in hook_entity_prepare_view to both of the referring items, observe how GenericFileFormatter makes a total hash of it: the first file link will get the attributes of the second and the second will get nothing.

Proposed resolution

          // Add the referring item, in case the formatter needs it.
          if ($entity->_referringItem) {
            $entity = clone $entity;
          }
          $entity->_referringItem = $items[$delta];
          $entity->_entityReferenceViewLangcode = $langcode;
          $entities[$delta] = $entity;

The conditional cloning is new.

Remaining tasks

Make a PR, add a test.

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet