Fix bug

Author: lunny

services/pull/merge.go

@@ -728,7 +728,7 @@ func SetMerged(ctx context.Context, pr *issues_model.PullRequest, mergedCommitID
 
 	// update merge ref, this is necessary to ensure pr.MergedCommitID can be used to do diff operations even
 	// if the repository rebased/force-pushed and the pull request's merge commit is no longer in the history
-	if err := git.UpdateRef(ctx, pr.BaseRepo.RepoPath(), pr.GetGitMergeRefName(), pr.MergedCommitID); err != nil {
+	if err := git.UpdateRef(ctx, pr.Issue.Repo.RepoPath(), pr.GetGitMergeRefName(), pr.MergedCommitID); err != nil {
 		return false, fmt.Errorf("UpdateRef: %w", err)
 	}