bug#56393: Actually fix the long lines display bug

[Lars Ingebrigtsen]

Would it be feasible to combine Gregory's work here with the new ticks
code?

That is, not enabling the new mode by default in all buffers, but if
we've registered that a particular window has generated a lot of ticks,
then we check the buffer for long lines and then enable the mode in that
buffer.

I.e., postpone enabling the mode until we see that we need it.  That
should make most programmatic usages not trigger the mode.

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