Don't run the diff action if the PR has been merged
Don't run the diff action if the PR has been merged

(This wasn't a problem until we started running the action on label
changes.)

Fixes #6777

jskeet/google-cloud-dotnet,jskeet/gcloud-dotnet,googleapis/google-cloud-dotnet,jskeet/google-cloud-dotnet,googleapis/google-cloud-dotnet,jskeet/google-cloud-dotnet,jskeet/google-cloud-dotnet,jskeet/google-cloud-dotnet,googleapis/google-cloud-dotnet