Re: change in behaviour for format_type() call

I wrote:
> I don't see anything in the commit message or linked discussion to
> indicate that any visible behavior change was intended, so I think
> you're right, this is a bug.  Will check and push your patch.

Actually, this patch still wasn't quite right: although it fixed
one aspect of the behavior, it still produced identical results
for typemod NULL and typemod -1, which as the function's comment
explains is not what should happen.  I tweaked the logic to look
as much as possible like before, and added a regression test.