Backport r1525902: issues #4428, #4429: external points 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/branches/1.7.x-issue4428@1525914 13f79535-47bb-0310-9956-ffa450edef68