bug#56393: Actually fix the long lines display bug

[Phil Sainty]

On 2022-07-07 01:45, Gregory Heytings wrote:
> I see, so-long restricts its action to so-long-target-modes,
> is that correct?

That's correct.  The so-long-predicate function determines
whether to take action, but the predicate is only called in
the first place if the buffer's major mode derives from one
of the target modes.