Follow #25229
Copy from
https://github.com/go-gitea/gitea/pull/26290#issuecomment-1663135186
The bug is that we cannot get changed files for the
`pull_request_target` event. This event runs in the context of the base
branch, so we won't get any changes if we call
`GetFilesChangedSinceCommit` with `PullRequest.Base.Ref`.