patch 9.1.1240: Regression with ic/ac text objects and comment plugin

patch 9.1.1240: Regression with ic/ac text objects and comment plugin

Problem:  Regression with ic/ac text objects and comment plugin
Solution: Fix regression, update tests (Maxim Kim)

fix regression: sometimes ic/ac should be line-wise

```
int main() {
    // multilple comments
    // cursor is between them
}

 # dac ->

int main() {
}
```

closes: #16947
closes: #16980

Signed-off-by: Maxim Kim <habamax@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>