[patch] psql tab completion for grant execute

Hi,

When tab-completing after GRANT EXECUTE, currently psql injects
"PROCEDURE", rather than the expected "ON".

The code for completing with "ON" is there, but it's not reached due to
falling earlier into another branch, one that handles CREATE TRIGGER.

A trivial patch is attached. It adds the condition that if EXECUTE is
preceded by GRANT itself preceded by nothing, then that completion
with PROCEDURE is skipped.

I've looked at fixing it more directly, by testing if the EXECUTE
is part of a CREATE TRIGGER, but it didn't seem fitting to go
looking backwards that many words into the string (more
than the 5 words suggested by the rest of the code).

Best regards,
--
Daniel Vérité
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