Support allowing no context lines to be printed with a diff.

* subversion/libsvn_diff/diff_file.c
  (svn_diff_file_output_unified4): Only default if negative.

* subversion/include/svn_diff.h
  (svn_diff_file_options_t): Update documented constraints on the context_size
    member to match the new behavior.
  (svn_diff_file_output_unified4): Update docs accordingly.
  (svn_diff_file_output_unified3): Make doc slightly clearer.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1618663 13f79535-47bb-0310-9956-ffa450edef68