Moved entity instance creation to separate method

This could be the first step to implement simple single table
inheritance. You can override the createEntityInstance() in your
derived Repository/Entity to create subclassed entities depending on some
row property value.

Anyway, it's good to extract entity creation functionalty into seperate
method, I believe.

Really, entity instantiation logic is common for both Repository and
Entity, so we should look for more DRY solution (use Mapper maybe?).