PullRequests *to* forked repo from the forked repo branch should be treated as normal PullRequest

Filed from https://github.com/reviewdog/action-shellcheck/issues/9

[isPRFromForkedRepo](https://github.com/reviewdog/reviewdog/blob/bfb8cbbf718172a6045f358bf81ac19542661524/cmd/reviewdog/doghouse.go#L253:6) should return false.