Fix brown paper bag bug in
bbe08b8869bd29d587f24ef18eb45c7d4d14afca.
We must issue the TRUNCATE command first and update relfrozenxid
and relminmxid afterward; otherwise, TRUNCATE overwrites the
previously-set values.
Add a test case like I should have done the first time.
Per buildfarm report from TestUpgradeXversion.pm, by way of Tom
Lane.