Fix for bug 7894 replaces a field(s) in a non-aggregate function with a item
reference if such a field was specified in the GROUP BY clause in order to
get a correct result.
When ROLLUP is involved this lead to a wrong result due to value of a such
field is got through a copy function and copying happens after the function
evaluation.
Such replacement isn't needed if grouping is also done by such a function.

The change_group_ref() function now isn't called for a function present in
the group list.


mysql-test/t/olap.test:
  Added the test case for the bug#20825:  rollup puts non-equal values together
mysql-test/r/olap.result:
  Added the test case for the bug#20825:  rollup puts non-equal values together
sql/sql_select.cc:
  Fixed bug#20825: rollup puts non-equal values together
  The change_group_ref() function now isn't called for a function present in
  the group list.