Revisit the infinite repr->str->repr recursion fix

[matthiask]

https://github.com/jazzband/django-debug-toolbar/commit/1a3cabac1c3d9b98b0d024a1bb907f20f487c5cf changed the testsuite to not test for the case it was supposed to test.

It is different now that str(form) uses the template engine again, so maybe we cannot simply go back to the old state.

Maybe _store_template_info should check its stack and look whether it is inside another _store_template_info call already, and terminate if it is so.