Extend cancel support to conflict output of svn_diff_file_output_merge3().

* subversion/libsvn_diff/diff_file.c
  (svn_diff3__file_output_baton_t): Add cancel argument members.
  (output_conflict): Switch to svn_diff_output2() using the cancel arguments
    from the baton.
  (svn_diff_file_output_merge3): Fill the cancel members of the baton.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1618863 13f79535-47bb-0310-9956-ffa450edef68