bug#46859: 28.0.50; [PATCH]: Add option to truncate long lines in xref.el

[Gregory Heytings]

> > Just tried 'exact, it works, it's slow (but not horribly slow: 1.2 
> > seconds on that 25 MB file)
>
> 1.2s is pretty slow for this purpose.

It is: (1) on a 25 MB file (not the typical case), (2) on a file with an 
exotic encoding (not a typical case either).  On typical files (UTF-8 or 
single byte encoding) the delay is not noticeable (a few milliseconds).

> Otherwise it would be possible to use this approach (by introducing a 
> Grep-specific location type) and avoid parsing the line contents for 
> every match.

It would be much faster, especially with very long lines, which was the 
question with which this bug report started.

> One downside would be missing on the syntax highlighting which we 
> sometimes get for already-fontified parts of buffers, but it's 
> unreliable anyway.

Yes, I don't understand why syntax highlighting is used in such buffers, 
IMO it's useless, it would be useful only if (1) it were reliable and (2) 
all lines were highlighted.