Issue title: Hide / Remove the "Unchanged files with check annotations " within a Pull Request
 Issue body: Hi,

Firstly, ReviewDog is such a great tool, and I'm really looking forward to adding it to our build pipelines.

I've been having a play with the following files:

issues.ts:
```
class HelloWorld {
  hello: string;
}

const something: string = '';

console.log('heya', 'hello world');
```

issues.txt
```
setupJest.ts:1:10: Hello World
issues.ts:1:10: Hello World
issues.ts:1:11: Some error
issues.ts:5:8: Something else
issues.ts:7:8: Hi there

```

Command:
`cat issues.txt | reviewdog -efm="%f:%l:%c: %m" -reporter=github-check -level=error`

The good news, is that it's showing the correct annotations in Github :)
![image](https://user-images.githubusercontent.com/1135454/116423863-f6f7c500-a838-11eb-9fce-e65e4910d3f5.png)

However, I'm seeing the Github "Unchanged files with check annotations" section, showing that a file outside the Pull Request has a issue.

![image](https://user-images.githubusercontent.com/1135454/116424637-96b55300-a839-11eb-98ce-553924383379.png)

Ideally, we'd like ReviewDog to only pick up files that have been changed in this PR.

Any ideas?


Thanks,










 Comments: 
 Comment 0: Changing the GitHub action to use `on: pull_request` and the reporter to `-reporter=github-pr-check` no longer shows the "unchanged files with check annotations" 
 Comment 1: I have the same issue but I don't want to switch to `github-pr-check`, any other ideas how to fix it?
 Comment 2: Is this related to reviewdog or to github? 
{'source': 'GithubIssues', 'repo_name_with_owner': 'reviewdog/reviewdog', 'issue_id': 870036643, 'repo_stars': 4879.0}