Data update pruning for template

This is a feature request for potential performance improvement. Maybe we can prevent updating `<template>` when all template data is not updated, for example
```typescript
this.setData({ C: { text: 'msg' } }}
```
will not trigger update process inside the following `<template is="tmpl">`
```html
<text>{{ C.text }}</text>
<template is="tmpl" data="{{ ...A, ...B }}" /> 
```