Since all current and foreseeable future command tags will be pure ASCII,
there is no need to do conversion on them.  This saves a few cycles and also
avoids polluting otherwise-pristine subtransaction memory contexts, which
is the cause of the backend memory leak exhibited in bug #5302.  (Someday
we'll probably want to have a better method of determining whether
subtransaction contexts need to be kept around, but today is not that day.)

Backpatch to 8.0.  The cycle-shaving aspect of this would work in 7.4
too, but without subtransactions the memory-leak aspect doesn't apply,
so it doesn't seem worth touching 7.4.