Never reuse dirty rendered markup when mounting

[syranide]

@zpao @sebmarkbage #1912 only emits a warning, it's still inconsistent the way we decide to reuse rendered markup, if the rendered markup is the first node then we do reuse, else we don't. I suggest we make it consistent and never reuse unless the rendered markup is alone.

Note: if approved, must not be merged before #1912 has been shipped in a release.

[gaearon]

I feel that the current behavior strikes a good enough balance of “working in most cases”. I’m not sure whether being more strict here would solve anyone’s problems so I’m inclined to close this as a stale PR unless there is more community or team support behind this. Thank you for contributing though! Please let me know if I’m missing something.