Fix handling of branches with @ character

Fixes https://github.com/rowanj/gitx/issues/233.

E.g.

![screen shot 2013-07-29 at 7 39 20 pm](https://f.cloud.github.com/assets/96004/873744/73c612bc-f87e-11e2-8eda-d4dd9f3b079f.png)