Skip closed merge requests where the change is already in trunk branch

Author: manicminer

project.go

@@ -285,7 +285,6 @@ func (p *project) migrateMergeRequests(ctx context.Context) {
 		} else {
 			successCount++
 		}
-		// call out to migrateMergeRequest and increment counters
 	}
 
 	skippedCount := totalCount - successCount - failureCount
@@ -572,6 +571,10 @@ func (p *project) migrateMergeRequest(ctx context.Context, mergeRequest *gitlab.
 			Draft:               &mergeRequest.Draft,
 		}
 		if pullRequest, _, err = gh.PullRequests.Create(ctx, p.githubPath[0], p.githubPath[1], &newPullRequest); err != nil {
+			if mergeRequest.State == "closed" && strings.Contains(err.Error(), "No commits between") {
+				logger.Debug("skipping closed merge request as the change is already present in trunk branch", "owner", p.githubPath[0], "repo", p.githubPath[1], "merge_request_id", mergeRequest.IID)
+				return nil
+			}
 			return fmt.Errorf("creating pull request: %v", err)
 		}