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

[Gregory Heytings]

> There's (at least) one more aspect of this, as long as Text mode is 
> being used: Text mode doesn't force bidi-paragraph-direction to be 
> left-to-right, whereas all descendants of prog-mode, including js-mode, 
> do.  Leaving bidi-paragraph-direction at nil means Emacs needs to 
> determine the base paragraph direction each time it's about to redisplay 
> a window, and that might be expensive, especially in a large buffer 
> without any paragraph breaks (by default, an empty line), because that 
> is determined by the first strong directional character of the 
> paragraph.  So for a more fair comparison with Text mode, you should set 
> bidi-paragraph-direction to the value left-to-right in text-mode 
> buffers.

Indeed, that seems to be the culprit here, I didn't know that text-mode 
was an exception here.  If I set bidi-paragraph-direction to 
'left-to-right after visiting the arabic-small.txt file, Emacs (mis) 
behaves like it does for the other Arabic files: it becomes slow, and C-n 
C-p do not work correctly anymore.