PullRequest.Fork field is unintended content

[kamijin-fanta]

It seems that the webhook payload is wrong when a Pull Request or Merge Request is issued within GitHub and GitLab.The PullRequest.Fork is always set to the repository name, even when the PR comes from a branch or fork.

I think the string should only be specified when the PR is issued from the fork repository, otherwise it should be empty.

github driver: https://github.com/drone/go-scm/blob/v1.15.2/scm/driver/github/pr.go#L153
gitlab driver: https://github.com/drone/go-scm/blob/v1.15.2/scm/driver/gitlab/webhook.go#L267
bitbucket driver: https://github.com/drone/go-scm/blob/v1.15.2/scm/driver/bitbucket/webhook.go#L585

It seems that the code that uses this library is also expected to specify an empty string if it is not forked repository.
https://github.com/drone/drone/blob/v2.4.0/plugin/secret/encrypted.go#L57