Danger is using the wrong set of commits when Pipelines for Merged Results is enabled

In #200037 (closed), we enabled Pipelines for Merged Results for this project. This means that pipelines operate on the merge of the source and target branch, rather than just on the target branch.

Unfortunately, this appears to have broken Danger: it's using all commits in the pipeline, rather than just those that are added by the target branch. For an example, see !26037 (comment 294971110). This MR didn't have any frontend changes, but one of the commits it diverges from master with was a406c5e6, which does contain frontend changes.

Before a rebase, this was even more noticeable: