Prevent replace_related from triggering lifecycle events

[gwafotapa]

Objective

EntityWorldMut::replace_related triggers Insert and Replace lifecycle events of the target. Additionally, depending on the input, it will also trigger Add and Remove. This happens when there is no intersection between the old and new relations because we start by removing the old before adding the new.

Triggering Add and Remove is clearly wrong. For Insert and Replace there are two perspectives:

• Either we think of replace_related as mutating the RelationshipTarget component in which case no lifecycle events should trigger;
• Or we think of replace_related as replacing the RelationshipTarget component in which case Insert and Replace should fire.

This PR goes with the first approach because:

• It feels more natural (kinda subjective);
• It is more consistent with replace_related_with_difference which doesn't fire any lifecycle events.

Solution

Building on the existing trick, we populate the dummy with the entity id of the target, ensuring that the RelationshipTarget component never gets emptied during the breaking of existing relations. This prevents Remove.

Finally we restore the correctly initialized RelationshipTarget component by mutating it directly instead of using EntityWorldMut::insert to avoid triggering Insert and Replace.

Testing

Added a test to ensure this works correctly.
Added a similar test for replace_related_with_difference.

[alice-i-cecile]

Really good explanation, comments and tests. Thank you.

[gwafotapa]

Minor tweak. No need to call mem::replace at the end since we don't need the dummy anymore. We can just mutate the RelationshipTarget component directly.