There's a great blog post on the subject, from Igor Zevaka
You first have to fetch the hidden pull request ref, and place it under the pr/NNN remote branch:
git fetch origin refs/pull/1234/head:refs/remotes/pr/1234
(replace 1234 by the pull request number)
After this it's just a classical rebase: checkout the pull request, and rebase it on your master branch:
git checkout pr/1234
git rebase master
or alternatively
git rebase master pr/1234
You might of course see conflicts.
Now the pull request is rebased on master, and is the current branch. If you want to update master just merge the pull request (we say --ff-only because we know it'll be a fast-forward merge):
git checkout master
git merge --ff-only pr/1234
To have a one-liner some shell aliasing is required, it's just a matter of scripting :)