Support -U option in diff utility.

Also removes deprication warning from using svn_diff_file_output_unified3().

* tools/diff/diff.c
  (do_diff): Use svn_diff_file_output_unified4().
  (main): Know to capture the argument following -U so
    svn_diff_file_options_parse() can capture the context size.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1618618 13f79535-47bb-0310-9956-ffa450edef68