前端：修复嵌套布局KeepAlive混乱的问题

[lauer3912]

// 关键源码
 if (mode === 'out-in') {
        // return placeholder node and queue update when leave finishes
        this._leaving = true
        mergeVNodeHook(oldData, 'afterLeave', () => {
          this._leaving = false
          this.$forceUpdate()
        })
        return placeholder(h, rawChild)
      }

// 最终代码
      // handle transition mode
      if (mode === 'out-in') {
        // return placeholder node and queue update when leave finishes
        this._leaving = true
        mergeVNodeHook(oldData, 'afterLeave', () => {
          this._leaving = false
          this.$forceUpdate()
        })
        if (!/\d-keep-alive$/.test(rawChild.tag)) {
          return placeholder(h, rawChild);
        }
      }