Update bb fast push unique

dirty 1000 ember data models:

before:
![screenshot 2014-08-20 01 04 53](https://cloud.githubusercontent.com/assets/1377/3977013/24ed30ac-2830-11e4-9046-1ef2337393f2.png)
![screenshot 2014-08-20 01 07 37](https://cloud.githubusercontent.com/assets/1377/3977027/5a02699c-2830-11e4-86ff-93d5c8a5c3c3.png)


after:

![screenshot 2014-08-20 01 04 48](https://cloud.githubusercontent.com/assets/1377/3977015/2aadd4f6-2830-11e4-8f94-5ea976e97e4c.png)
![screenshot 2014-08-20 01 07 31](https://cloud.githubusercontent.com/assets/1377/3977030/5e43bb28-2830-11e4-9ddf-bef627e62877.png)

Another pathological churn example:

before:

![screenshot 2014-08-20 01 08 55](https://cloud.githubusercontent.com/assets/1377/3977037/88be51f6-2830-11e4-8909-582b2762d74e.png)

after:
![screenshot 2014-08-20 01 09 01](https://cloud.githubusercontent.com/assets/1377/3977040/9342f924-2830-11e4-9081-6cea52bf70ed.png)