Support multi parents merge

Support multi parents merge

![Merge](https://f.cloud.github.com/assets/2804967/80898/260af344-62c9-11e2-90b3-d7f09ad2cb40.png)

![MergeParentList](https://f.cloud.github.com/assets/2804967/80899/26098d92-62c9-11e2-99f3-fe8b29e445fb.png)