Fixed the results of a test for group_concat.
  After the fix foor bug #11639 the results became
  correct.
olap.result, olap.test:
  Added a test case for bug #11639.
sql_select.cc:
  Fixed bug #11639: a wrong result set when using a view
  instead of the underlying table in a rollup query 
  executed through filesort.
  The old code did not take into account that we always
  use an Item_ref object when referring to a view column.
item.h:
  Fixed bug #11639.
  Now if two Item_ref items ref1 and ref2 refer to the same field
  then ref1->eq(ref2) returns 1.