The idea behind a branch for a pull request is to allow for said branch to be automatically deleted once the pull request is accepted.
And since April 2013, that branch will be deleted for you:
You are then suppose to update/rebase your master from the master of the upstream repo in order to get what you developed in isolation a branch from the official repo that you have forked (since that repo has accepted your pull request)
The name of the branch should represent the development effort you are engaged in.