For opening a PR, usually you can just fork the repo and create a branch in the forked repo, and then open a PR. It is not necessary to create a branch in the original repo only for a PR. You can open a branch for a big feature, which usually needs a lot of commits and also a very long time before merge it back to master.
At one point, the PPMC agreed (without a vote) that all committers must follow the same processes as all other contributors, that they must not abuse their write privileges by circumventing the process that every other contributor must follow. The typical contributor cannot create a branch in the repository for the person contributions and neither should committers.
Greg
