Extend cancel support into the conflict output.

* subversion/libsvn_diff/diff_memory.c
  (merge_output_baton_t): Add cancel support.
  (svn_diff_mem_string_output_merge3): Fill merge_output_baton_t cancel members.
  (output_conflict): Use svn_diff_output2() with cancel support from the baton.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1618842 13f79535-47bb-0310-9956-ffa450edef68