bug#56682: feature/improved-locked-narrowing 9dee6df39c: Reworked locked narrowing.

[Juri Linkov]

>> For example, when there is at least one long line near the end of
>> a diff-mode buffer, then sometimes the buffer gets truncated,
>> 'diff-hunk-file-names' returns nil on diff headings,
>> and there are a lot of such errors in the Messages buffer:
>>
>>   Error during redisplay: (jit-lock-function 1) signaled (args-out-of-range 
>> #<buffer *vc-diff*> 1118 1370)
>>   ...
>>
>> Maybe it's still possible to fix this?
>
> We can and should fix this and other problems, but only if they are
> reported.  Would you please file a bug report for any such problem you
> find?

I noticed this problem only in one commit last week, but didn't realize
it's caused by long lines.  I had time to debug it only today.
Now could try to create a minimal test case.