Exclude inline admins from being reverted when not followed.

Fixes etianen/django-reversion#222

I put the check inside of _hack_inline_formset_initial so the logic would only have to be in one place.

I couldn't find a way to accurately test the actual reversion from the admin since what is submitted would have the delete value I specify, not the delete value reversion generated. I believe my method of checking the initial page load for the value of delete should work but if you have a better idea, please let me know.