Today looking at the Travis log of a Pull request build I saw this interesting command:
Turns out that GitHub makes available from your main remote the PR branches as remote refs.
Also (discovered by blind guessing), if you change
/head you get a ref to the clean PR head, unmerged with its target branch. What can be the most useful is up to you, I guess.
This is probably easy because GH on its side stores all the forks of a repo as the same Git repository.
An example in the Coderwall ProTip linked at the title.