Make 'svn diff' sort property change output on a specific targets by their
property name. This makes the diff output more stable between runs, as
otherwise their order will depend on the hash ordering when calculating
the changes.

* subversion/include/private/svn_diff_private.h
  (svn_diff__display_prop_diffs): Rename argument.

* subversion/libsvn_diff/util.c
  (includes): Add private/svn_sorts_private.h.
  (propchange_sort): New function.
  (svn_diff__display_prop_diffs): Rename pool argument. Sort properties before
    printing them.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1589360 13f79535-47bb-0310-9956-ffa450edef68