improve: the readability of updateChildren

[gebilaoxiong]

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• Bugfix
• Feature
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• Yes
• No

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[gebilaoxiong]

Well :

Although elmToMove.elm and newStartVnode.elm are the same thing

But :

When the element index of the old list changes

We operate the old list：

Such as : https://github.com/vuejs/vue/blob/dev/src/core/vdom/patch.js#L389

else if (sameVnode(oldStartVnode, newEndVnode)) { // Vnode moved right
  // ...
  canMove && nodeOps.insertBefore(parentElm, oldStartVnode.elm, nodeOps.nextSibling(oldEndVnode.elm))
  // ...
}

And : https://github.com/vuejs/vue/blob/dev/src/core/vdom/patch.js#L394

else if (sameVnode(oldEndVnode, newStartVnode)) { // Vnode moved left
  // ...
  canMove && nodeOps.insertBefore(parentElm, oldEndVnode.elm, oldStartVnode.elm)
  // ...
}

I think updateChildren is talking about:

Operation the old list, let it become to the new one

So I think there should be use elmToMove.elm, It can make the code easier to understand