Make the single file merge code use the diff processor instead of calling
the implementation directly.

* subversion/libsvn_client/merge.c
  (do_file_merge): Call the diff processor instead of calling the
    implementation directly.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1440432 13f79535-47bb-0310-9956-ffa450edef68