function installComponentHooks (data: VNodeData) {
  const hooks = data.hook || (data.hook = {})
  for (let i = 0; i < hooksToMerge.length; i++) { //["init", "prepatch", "insert", "destroy"]
    const key = hooksToMerge[i]
    const existing = hooks[key]
    const toMerge = componentVNodeHooks[key]
    if (existing !== toMerge && !(existing && existing._merged)) {
      hooks[key] = existing ? mergeHook(toMerge, existing) : toMerge
    }
  }
}
正常来说toMerge变量是一直为undefined的，什么情况下会走到有值的逻辑了？

function mergeHook (f1: any, f2: any): Function {
  const merged = (a, b) => {
    // 这里可能出现两个init钩子函数，为什么都运行一次就算合并了呢？
    f1(a, b) 
    f2(a, b)
  }
  merged._merged = true
  return merged
}