Fix breakage of "\pset format latex".

Fix breakage of "\pset format latex".

Commit eaf746a5b unintentionally made psql's "latex" output format
inaccessible, since not only "latex" but all abbreviations of it
were considered ambiguous against "latex-longtable".  Let's go
back to the longstanding behavior that all shortened versions
mean "latex", and you have to write at least "latex-" to get
"latex-longtable".  This leaves the only difference from pre-v12
behavior being that "\pset format a" is considered ambiguous.

The fact that the regression tests didn't expose this is pretty bad,
but fixing it is material for a separate commit.

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