hub merge PULLREQ-URL
This creates a local merge commit in the current branch, but does not actually change the state of the pull request. However, the pull request will get auto-closed and marked as "merged" as soon as the newly created merge commit is pushed to the default branch of the remote repository.
$ hub merge https://github.com/jingweno/gh/pull/73 > git fetch origin refs/pull/73/head > git merge FETCH_HEAD --no-ff -m "Merge pull request #73 from jingweno/feature..."
hub-checkout(1), hub(1), git-merge(1)
hub(1), hub-checkout(1), hub-pull-request(1).