Implement cancellation in a bit of libsvn_client diff/merge handling to avoid
using deprecated functions... and to allow cancelling.

* subversion/svn/cl.h
  (svn_cl__merge_file): Add cancel arguments and move single output argument
    to the start of the argument list.

* subversion/svn/conflict-callbacks.c
  (show_diff): Add cancel arguments. Update caller.
  (handle_text_conflict): Update caller.

* subversion/svn/file-merge.c
  (svn_cl__merge_file): Add cancel arguments. Move output argument.
    Update caller.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1582460 13f79535-47bb-0310-9956-ffa450edef68