Make psql's "\pset format" command reject non-unique abbreviations.

Make psql's "\pset format" command reject non-unique abbreviations.

The previous behavior of preferring the oldest match had the advantage
of not breaking existing scripts when we add a conflicting format name;
but that behavior was undocumented and fragile (it seems just luck that
commit add9182e5 didn't break it).  Let's go over to the less mistake-
prone approach of complaining when there are multiple matches.

Since this is a small compatibility break, no back-patch.

Daniel Vérité

Discussion: https://postgr.es/m/cb7e1caf-3ea6-450d-af28-f524903a030c@manitou-mail.org