Support pull_request_target event type

[Fishbowler]

The pull_request_target event is an alternative to the pull_request event but with some different security considerations. Docs.

We use it on private repositories to allow dependabot access to our secrets, safe in the knowledge that a private repo can't expose our secrets to drive-by malicious PRs.

This is a simple change that extends the logic for "pull_request" to "pull_request_target" for comparisons of changes, etc.