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

[ndame]

> For keep-lines, if
> matches don't span lines, then the number of lines is equal to the
> number of lines kept, rather than lines deleted. And there is less
> usefulness in reporting lines kept,

I agree, that's why I suggested keep-lines should also count and report
the number of deletions performed which can be useful info.

This is easy to do by incrementing a counter at the two delete-region calls
in keep-lines.