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

[ndame]

> 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.

keep-lines has two delete-region calls in two separate cases. Even counting only
these deletions and saying "x deletions were performed" is more informative than
saying nothing. It doesn't have to say "number of lines".

I guess most users use it to remove whole lines most of the time, so the match
is usually not split across lines, therefore the number of deletions in these cases is
equal to the number of deleted lines.