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
