Fix poster is not loaded in get default merge message (#27657) (#27666)

Backport #27657 by @lunny

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

services/pull/merge.go

@@ -44,6 +44,9 @@ func getMergeMessage(ctx context.Context, baseGitRepo *git.Repository, pr *issue
 	if err := pr.LoadIssue(ctx); err != nil {
 		return "", "", err
 	}
+	if err := pr.Issue.LoadPoster(ctx); err != nil {
+		return "", "", err
+	}
 
 	isExternalTracker := pr.BaseRepo.UnitEnabled(ctx, unit.TypeExternalTracker)
 	issueReference := "#"