Change all occurrences of 'diff_cmd_baton' to 'dwi'.

* subversion/libsvn_client/diff.c
  (just_paths_for_diff_labels),
  (diff_props_changed),
  (svn_client_diff6),
  (svn_client_diff_peg6): changed local variable diff_cmd_baton to
    dwi throughout.

Approved by: rhuijben


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1602122 13f79535-47bb-0310-9956-ffa450edef68