varnish: mangle request data for uncacheable objects

Uncacheable objects in bits now receive an hit_for_pass response from
vcl_fetch; so we need to mangle the request url there or subsequent
requests will be sent un-rewritten to the backend, resulting in a 404
which will be cached for a long time. This only affected urls with
debug=true (not cached).

Bug #70557

Change-Id: If333663b23e50b37c09412ab55d28704b018e58c
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>