On the invoke-diff-cmd branch: Raise an error if both diff_cmd and
invoke-diff-cmd are set.

* subversion/include/svn_error_codes.h

  (SVN_CLIENT_DIFF_CMD): New macro.

* subversion/libsvn_client/diff.c 

  (diff_content_changed): Raise an error if both diff_cmd and
    invoke-diff-cmd are set.

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/invoke-diff-cmd-feature@1488813 13f79535-47bb-0310-9956-ffa450edef68