Data update pruning for template

[SgLy]

This is a feature request for potential performance improvement. Maybe we can prevent updating <template> when all template data is not updated, for example

this.setData({ C: { text: 'msg' } }}

will not trigger update process inside the following <template is="tmpl">

<text>{{ C.text }}</text>
<template is="tmpl" data="{{ ...A, ...B }}" />