Fix issues #4428 and #4429: external pointing at redirected URL.

* subversion/libsvn_client/externals.c
  (handle_external_item_change):
    Pass the possibly redirected URL to switch_dir_external(), not the old one.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1525902 13f79535-47bb-0310-9956-ffa450edef68