minor fix for acquire_inherited_sample_rows

Hi.

acquire_inherited_sample_rows() currently uses equalTupleDescs() being
false as the condition for going to tupconv.c to determine whether tuple
conversion is needed.  But equalTupleDescs() will always return false if
it's passed TupleDesc's of two different tables, which is the most common
case here.  So I first thought we should just unconditionally go to
tupconv.c, but there is still one case where we don't need to, which is
the case where the child table is same as the parent table.  However, it
would be much cheaper to just check if the relation OIDs are different
instead of calling equalTupleDescs, which the attached patch teaches it to do.

Thanks,
Amit