Do not add do not merge label when PR is merged

This commit fixes the issue described in #23205. Release note plugin
should not label the PR when it is already merged.

Signed-off-by: Jakub Guzik <jguzik@redhat.com>

prow/plugins/releasenote/releasenote.go

@@ -275,7 +275,7 @@ func handlePR(gc githubClient, log *logrus.Entry, pr *github.PullRequestEvent) e
 			}
 			if containsNoneCommand(comments) {
 				labelToAdd = labels.ReleaseNoteNone
-			} else if !prLabels.Has(labels.ReleaseNoteLabelNeeded) {
+			} else if !prLabels.Has(labels.ReleaseNoteLabelNeeded) && !pr.PullRequest.Merged {
 				comment := plugins.FormatSimpleResponse(pr.PullRequest.User.Login, releaseNoteBody)
 				if err := gc.CreateComment(org, repo, pr.Number, comment); err != nil {
 					log.WithError(err).Errorf("Failed to comment on %s/%s#%d with comment %q.", org, repo, pr.Number, comment)

prow/plugins/releasenote/releasenote_test.go

@@ -592,6 +592,7 @@ func TestShouldHandlePR(t *testing.T) {
 		name           string
 		action         github.PullRequestEventAction
 		label          string
+		pr             github.PullRequest
 		expectedResult bool
 	}{
 		{
@@ -606,6 +607,11 @@ func TestShouldHandlePR(t *testing.T) {
 			label:          "",
 			expectedResult: true,
 		},
+		{
+			name:           "Pull Request Action: Release Note label not needed when PR merged",
+			pr:             github.PullRequest{Merged: true},
+			expectedResult: false,
+		},
 		{
 			name:           "Pull Request Action: Release Note label",
 			action:         github.PullRequestActionLabeled,
@@ -621,13 +627,14 @@ func TestShouldHandlePR(t *testing.T) {
 	}
 
 	for _, test := range tests {
-		pr := github.PullRequestEvent{
+		pre := github.PullRequestEvent{
 			Action: test.action,
 			Label: github.Label{
 				Name: test.label,
 			},
+			PullRequest: test.pr,
 		}
-		result := shouldHandlePR(&pr)
+		result := shouldHandlePR(&pre)
 
 		if test.expectedResult != result {
 			t.Errorf("(%s): Expected value to be: %t, but got %t.", test.name, test.expectedResult, result)