bug#56682: Fix the long lines font locking related slowdowns

[Gerd Möllmann]

> Gerd Möllmann <gerd.moellmann@gmail.com> writes:
>
>> Font-lock       Output
>> ------------------------------------------------------------
>> on              GCs: 14 Elapsed time: 7.880788 seconds
>   on, and large gc-const-threshold
>                   GCs: 0 Elapsed time: 7.720531 seconds
>> off             GCs: 2 Elapsed time: 0.885791 seconds

I've profiled this.  The result looks sensible, but I have to add that I
had mixed results with profiling optimized builds, so maybe...  Just
saying.

Sorry for posting an image.  I haven't found a way to get that info from
Instruments as text (pointers welcome).