Also set debug_query_string.

Oversight in commit 9da0cc35

Peter Geoghegan, per a report by Phil Florent.

Discussion: https://postgr.es/m/CAH2-Wzmf-34hD4n40uTuE-ZY9P5c%2BmvhFbCdQfN%3DKrKiVm3j3A%40mail.gmail.com