bug#61215: 29.0.60; font-lock broken in diff-mode with long lines

[Juri Linkov]

>> 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)
>
> 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?

This is a new bug report created from bug#56682.