In the local-local diff handling, only produce file changed reports if the
file has really changed. When using the builtin diff the result is the same
as the diff library won't find any changes.

* subversion/libsvn_client/diff.c
  (explanation): Update to 1.8 truth.

* subversion/libsvn_client/diff_local.c
  (includes): Add svn_subst.h
  (do_arbitrary_files_diff): Only tell that a file is modified if it
    is modified. Not for every existing file.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1428416 13f79535-47bb-0310-9956-ffa450edef68