Hotpatch for infinite redirection-following loop seen processing URLs to http://cloju...

Hotpatch for infinite redirection-following loop seen processing URLs to http://clojure.org/ -- if we end up with an unstable redirect target (final item in a redirect chain ends up redirecting us somewhere else when we visit it again), just save the last version we saw instead of trying to start over.

Pretty much everything in File and File_redirection initial processing needs to be rewritten to be non-awful; this code is very hard to follow and very easy to make huge bugs. A fair amount of the complication is probably obsoleted by the redirection following being built into HTTPClient now.