Fix BranchHeads to work with branch2*

As of [this post](http://selenic.com/pipermail/mercurial-devel/2013-November/054749.html) the new format should be:

```
<branch head hex node> <open/closed state> <branch name>
```

Ran into this problem on Mercurial 3.0.2.

This fix is backwards compatible for older Mercurial versions.