BEIJING, Dec 18- Chinese internet giant Tencent Holdings Ltd said on Monday it would lead an $863 million investment in apparel platform Vipshop Holdings Ltd, upping its rivalry in retail with Alibaba Group Holding Ltd.. Tencent will invest $604 million in exchange for a 7 percent stake in Vipshop, while e-commerce firm JD.com Inc- a long-standing ally- will...