Performance improvements for pull request list API (#30490)

Fix #30483

---------

Co-authored-by: yp05327 <576951401@qq.com>
Co-authored-by: Giteabot <teabot@gitea.io>

models/issues/assignees.go

@@ -27,23 +27,27 @@ func init() {
 
 // LoadAssignees load assignees of this issue.
 func (issue *Issue) LoadAssignees(ctx context.Context) (err error) {
+	if issue.isAssigneeLoaded || len(issue.Assignees) > 0 {
+		return nil
+	}
+
 	// Reset maybe preexisting assignees
 	issue.Assignees = []*user_model.User{}
 	issue.Assignee = nil
 
-	err = db.GetEngine(ctx).Table("`user`").
+	if err = db.GetEngine(ctx).Table("`user`").
 		Join("INNER", "issue_assignees", "assignee_id = `user`.id").
 		Where("issue_assignees.issue_id = ?", issue.ID).
-		Find(&issue.Assignees)
-	if err != nil {
+		Find(&issue.Assignees); err != nil {
 		return err
 	}
 
+	issue.isAssigneeLoaded = true
 	// Check if we have at least one assignee and if yes put it in as `Assignee`
 	if len(issue.Assignees) > 0 {
 		issue.Assignee = issue.Assignees[0]
 	}
-	return err
+	return nil
 }
 
 // GetAssigneeIDsByIssue returns the IDs of users assigned to an issue