Specialize MemoryContextMemAllocated().
An AllocSet doubles the size of allocated blocks (up to maxBlockSize),
which means that the current block can represent half of the total
allocated space for the memory context. But the free space in the
current block may never have been touched, so don't count the
untouched memory as allocated for the purposes of
MemoryContextMemAllocated().
Discussion: https://postgr.es/m/
ec63d70b668818255486a83ffadc3aec492c1f57.camel@j-davis.com