validate-files.py: Use decode on output data from subprocesse.

This is necessary to be compatable with Python 3 which returns bytes instead
of strings for these calls.

* tools/hook-scripts/validate-files.py
  (Commands.svnlook_changed): Use decode on the lines and we read them
    and decode before writing stderr back out in case of an err.
  (Commands.user_command): Use decode before returning the stderr output.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1429453 13f79535-47bb-0310-9956-ffa450edef68