Fix incorrect object passed to the view when rendering the history compare action

[tamcy]

Subject

Currently, when rendering the entity compare view in the history action, only the current (first) entity object will be passed to the view, even when it is going to render the value of the comparing (secondary) entity object. This means that the field block of the show field template will receive the first entity (the base object) along with the secondary value (value to compare) when the secondary value is going to be rendered. This can cause problem if the template does not solely rely on the value but also the object for the correct output. This PR fixes this.

I am targeting this branch, because it is backward-compatible. Note that after applying the patch, the block field_compare in template base_show_field.html.twig will assume the the existence of the object_compare variable. This may cause compatibility problem. But given that

• SonataAdminExtension class is annotated @final, and
• The documentation states that show_field views should extend @SonataAdmin/CRUD/base_show_field.html.twig and should not contain a field_compare block

I think it is still reasonable to say that the patch won't break BC. Otherwise I'd suggest base_show_field.html.twig to be changed to something like this:

            {% if(object_compare is defined) %}
                {% set object = object_compare %}
            {% endif %}

Changelog

### Fixed
- Incorrect object being passed to the view when rendering the history compare action

[VincentLanglet]

Thanks @tamcy