Hardly the best place for a conversation, but... :) 

```
git checkout -b some-branch-to-work-in master
git pull git@github.com:onnokort/libopencm3.git remote-branch-you-want
test away :)
```

This is what's actually shown to repo collabs on the merge tab, and I've got an outstanding request in with github to show the instructions for everyone on all pull requests.

The magic refs path to get to pull requests is cool too, but hard to remember :|