Added a test in connection with the fix for
  bug #6106.
view.result, view.test:
  Added test cases for bugs #6106/6107.
sql_show.cc:
  The addition of the case for items of the type REF_ITEM in the
  function uses_only_table_name_fields became necessary after
  the fix for bug #6106.
sql_base.cc:
  The problem was due to the fact that two different column
  references were glued together though one of them belonged to
  a subquery while another to an outer query. This caused
  eventually a wrong calculation of values for the used_tables
  attribute.