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/306015/3899584/b8a0a706-227d-11e4-8a08-cb973f9bfc78.png)