bug#39908: 26.2; flush-lines and keep-lines should say how many lines were flushed

[Juri Linkov]

>  >  Upgrade to Emacs 27, this happens there already.
>
> I checked the news, but it only mentions flush-lines.
>
> It is also useful for keep-lines to say how many lines it removed, instead
> of saying nothing.

keep-lines can't say how many lines it removed, because it doesn't know the 
number.
And reimplementing it to count the number of removed lines will make it slower.