Doing multiple changes inside `transaction()` results multiple invocations of observer subscribed via `subscribe()`.

Doing multiple changes inside `transaction()` results multiple invocations of observer subscribed via `subscribe()`.

How to reproduce:

``` javascript
import 'https://unpkg.com/sinuous@0.27.12/dist/observable.js'
const {observable, subscribe, transaction, root}= self.observable
root(() => {
const a = observable(1)
const b = observable(2)
const log = []
subscribe(() => {
log.push(a() + b())
})
transaction(() => {
a(10)
b(20)
})
// Expected 3;30
// Got 3;12;30
console.assert(log.join(';') === '3;30')
})
```

[Link to https://CodePen.io/j-sen/pen/OJjNKVK](https://codepen.io/j-sen/pen/OJjNKVK)

Reproduce the example from the issue using the pinned sinuous@0.27.12 dist/observable.js build, focusing on the subscribe() and transaction() entry points. The behavior is corrected when the transaction logs only 3;30 instead of 3;12;30.

Written by the indexing model from the issue text.