fix #8041: templates inside v-pre should be rendered to HTML

[kball]

close #8041

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:

I don't believe this is breaking as content inside of v-pre is considered to be static. However it does change the outputted static content in the case where a <template> tag is labeled with v-pre or inside of a v-pre element.

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

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

I pushed a 'compiled' version of this to a different branch to show the working behavior on jsFiddle. The original fiddle from issue #8041: https://jsfiddle.net/zct418xa/1/
A fiddle with the updated behavior using vue.js compiled from this change: https://jsfiddle.net/f7nv9218/

[kball]

Any feedback on this PR? No pressure as I know how hard it is as a maintainer - I'm just excited to learn more about the Vue codebase and tackling issues is the best way I know to do that.

Will happily take feedback on approach, style, or anything else.

[zippaaa]

It is strange that they do not approve this changes.

Component

<template>
    <div>
        <template v-pre> <---- It's my template
            bla-bla            I need it later.
        </template>            But vue break it.

        <ul>
           <li>Hello
        </ul>
    </div>
</template>

[michael-hawker]

@kball @yyx990803 has this regressed?

If I create a new fiddle it still repros. Or is this change not in 2.6.10? I'm encountering this issue as posted here, so I want to know how I can get a version with this fix.

Thanks!

[cutPicturesMan]

@alekseymvt @michael-hawker see this issue: #10717