git: add alias to fetch GitHub Pull Requests
git: add alias to fetch GitHub Pull Requests

`git gh-pr 123`: fetch pull request 123 from `origin`, create a branch
called `gh-pr/123` and then check it out.

`git gh-pr 123 different-remote` will do the same but get the pull
request from the specified remote.

bronxct1/dotfiles,SBoudrias/dotfiles,saclark/dotfiles,jmyser/dotfiles,bronxct1/dotfiles,seiryu9/dotfiles,necolas/dotfiles,pmacom/dotfiles,sdillingham/dotfiles,SiTaggart/dotfiles,pertoyra/dotfiles,codfish/dotfiles,marc0uk/dotfiles,lgaechter/dotfiles,seiryu9/dotfiles,codfish/dotfiles