[WIP] Add Pick Branch Mode into Rebase dialog


![pick_branch](https://cloud.githubusercontent.com/assets/5831804/4869061/6115feba-6147-11e4-9f47-3ea7e71c59a4.png)

Testing Case:
- [X] Fetch & Rebase
  + Fetch
    + Use Arbitrary URL
    + Use Remote Branch
    + Launch Rebase
  + Rebase
    + Use Reverse
    + Branch: FETCH_HEAD (**None FF**)
    + Upstream: master (current branch)
- [ ] Fetch & Rebase
  + Fetch
    + Use Arbitrary URL
    + Use Remote Branch
    + Launch Rebase
  + Rebase
    + Use Reverse
    + Branch: FETCH_HEAD (**FF**)
    + Upstream: master (current branch)
- [ ] ...