Specifically, it is only equivalent to `--count-matches` when the
pattern(s) given can match over multiple lines.
We could have instead made `--multiline --count` always equivalent to
`--multiline --count-matches`, but this seems plausibly less useful.
Indeed, I think it's generally a good thing that users can enable
`-U/--multiline` but still use patterns that only match a single line.
Changing how that behaves would I think be more surprising.
Either way we slice this, it's unfortunately pretty subtle.
Fixes#2852
c45ec16360