bug#34520: delete-matching-lines should report how many lines it deleted

[積丹尼 Dan Jacobson]

delete-matching-lines is an alias for ‘flush-lines’ in ‘replace.el’.

It works great... or does it? Can't often tell. That's because well,
if there are matching lines off the screen, you won't really know, so
you have to go down there to have a look... and well, need eagle eyes
often too depending on the pattern and how many similar lines there
still are.

Got an idea!: Simply keep a count of how many lines were deleted, and
report that in the minibuffer, if using interactively.

What if there were no matching lines?
Then say
Deleted 0 matching lines.
Or beep "No matching lines!"

Try this with matching lines all below the visible part of the buffer:
C-x h [mark-whole-buffer]
M-x delete-matching-lines zzzzz

See the feeling (felt nothing, did it work in the first place one wonders?)