Add Entry will only show up when cycle.is_current

b/16702091
I'm not sure of any case where `parent_instance` wouldn't be a non-stub instance, so shouldn't need `parent_instance.reify`.  However, this would be better using `#using`, which ensure the object is loaded, e.g.:

```
{{#using cycle=parent_instance.cycle}}
{{#if cycle.is_current}}
...
{{/if}}
{{/using}}
```