Detect properly branches that contain `/`

Reported by https://twitter.com/lennym/status/497767552081494016. Users seem to have branch names containing `/` in them.

With this PR, gh parses properly those:

![image](https://cloud.githubusercontent.com/assets/306015/3899215/308438d8-2278-11e4-879c-1d5a79e99a44.png)