Resolve some deprecation warnings in libsvn_client/externals.c.
Resolve some deprecation warnings in libsvn_client/externals.c.

* subversion/libsvn_client/externals.c
  (switch_file_external): Use svn_wc_add4() and svn_wc_revert4().
   While here, use an absolute path to open a file.
  (resolve_relative_external_url): Use svn_uri_canonicalize().
  (handle_external_item_change,
   handle_external_item_change_wrapper): Use svn_dirent_join().


git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@879682 13f79535-47bb-0310-9956-ffa450edef68

YueLinHo/Subversion,wbond/subversion,wbond/subversion,wbond/subversion,wbond/subversion,wbond/subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion,YueLinHo/Subversion