Fixed #23682 -- Enhanced circular redirects detection in tests.

When the test client detects a redirect to a URL seen in the
currently followed chain it will now raise a RedirectCycleError
instead of just returning the first repeated response.

It will also complain when a single chain of redirects is longer
than 20, as this often means a redirect loop with varying URLs,
and even if it's not actually one, such long chains are likely
to be treated as loops by browsers.

Thanks Preston Timmons, Berker Peksag, and Tim Graham for reviews.

docs/releases/1.8.txt

@@ -799,6 +799,10 @@ Miscellaneous
   when both the ``fields`` and ``form_class`` attributes are specified.
   Previously, ``fields`` was silently ignored.
 
+* When following redirects, the test client now raises
+  :exc:`~django.test.client.RedirectCycleError` if it detects a loop or hits a
+  maximum redirect limit (rather than passing silently).
+
 .. _deprecated-features-1.8:
 
 Features deprecated in 1.8