CAN RETURN WRONG RESULT SET

PROBLEM
-------
In ha_partition::cmp_ref() we were only calling the 
underlying cmp_ref() of storage engine if the records
are in the same partiton,else we sort by partition and
returns the result.But the index merge intersect 
algorithm expects first to sort by row-id first and 
then by partition id.

FIX
---
Compare the refernces first using storage engine cmp_ref
and then if references are equal(only happens if 
non clustered index is used) then sort it by partition id.

[Approved by Mattiasj #rb3755]
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