Problem: GROUP_CONCAT(DISTINCT BIT_FIELD...) uses a tree to store keys;
which are constructed using a temporary table fields,
see Item_func_group_concat::setup().
As a) we don't store null bits in the tree where the bit fields store parts 
of their data and b) there's no method to properly compare two table records
we've got problem.

Fix: convert BIT fields to INT in the temporary table used.