Fix go-gitea#9552: Merge commits generated by pull request capture pu…

…ll request details (go-gitea#9635)

models/pull.go

@@ -175,7 +175,11 @@ func (pr *PullRequest) GetDefaultMergeMessage() string {
 			return ""
 		}
 	}
-	return fmt.Sprintf("Merge branch '%s' of %s/%s into %s", pr.HeadBranch, pr.MustHeadUserName(), pr.HeadRepo.Name, pr.BaseBranch)
+	if err := pr.LoadIssue(); err != nil {
+		log.Error("Cannot load issue %d for PR id %d: Error: %v", pr.IssueID, pr.ID, err)
+		return ""
+	}
+	return fmt.Sprintf("Merge pull request '%s' (#%d) from %s/%s into %s", pr.Issue.Title, pr.Issue.Index, pr.MustHeadUserName(), pr.HeadBranch, pr.BaseBranch)
 }
 
 // GetCommitMessages returns the commit messages between head and merge base (if there is one)