Following up on r1603847, add help output for -U diff option.

* subversion/svn/svn.c
  (svn_cl__options):

* subversion/svnlook/svnlook.c
  (options_table): Document - U.

* subversion/tests/cmdline/getopt_tests_data/svn_help_log_switch_stdout
  Tweak expected output.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1603871 13f79535-47bb-0310-9956-ffa450edef68