Add support for translating files from the 'arbritrary node diff'.

This hides many non changes such as keyword changes when performing
a local diff like
$ svn diff --old=trunk --new=branches/log-message-templates

* subversion/libsvn_client/diff_local.c
  (includes): Add svn_subst.h.
  (translate_if_necessary): New function.
  (do_file_diff): Call translate_if_necessary.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1623820 13f79535-47bb-0310-9956-ffa450edef68