fix(ref): preserve ref on components after removing root element (#6632, #6641)

[javoski]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

Other information:
fix #6632 & #6641 .

[TomKaltz]

+1

[nemtsov]

Just a heads up that this turned out to break our tests. Not a big deal, but just wanted to let you know. Here's what our code & tests looked like:

// Parent
<template>
  <div>
    <Child ref="child" />
  </div>
</template>

// Child
<template>
  <div v-if="$store.state.isOn">yes</div>
</template>


// test
expect(this.vm.$refs.child).toExist();

this.vm.$store.state.isOn = false;
await this.vm.$nextTick();

expect(this.vm.$refs.child).not.toExist();