For anyone out there on Github projects where devs work on forks and submit PR's to a central repo: This is useful if you want to try out someone's PR from Github without fetching their fork (and all their branches):
1.) In ~/.gitconfig
add these lines (if you don't have any aliases already setup):
[alias]
pr = !git fetch -fu ${2:-REMOTE_REPO_NAME} refs/pull/$1/head:pr/$1 && :
2.) Replace REMOTE_REPO_NAME
with the remote name for your project's central repo of course; PR's don't exist on your fork, but on the canonical repository.