bug#23278: 25.0.92; font-lock-ensure is too slow

[Eli Zaretskii]

> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Tue, 12 Apr 2016 22:49:09 +0300
> 
> Example:
> 
> Open xdisp.c, navigate to the first line. Evaluate:
> 
> (load "jit-lock")
> (benchmark 1000 '(font-lock-ensure (line-beginning-position)
>                                    (line-end-position)))
> 
> => 0.66s
> 
> Which puts a dampener on the idea of syntax highlighting xref results in
> the open files.
> 
> With the patch below, the benchmark executes 100 times faster.
> OK to apply?

If you run with this since the time of your message and saw no
problems, please push to emacs-25.  Otherwise, to master.

Thanks.