For those, lazy enough to open github after a fix on a local branch to create a pull request

brew install hub

or

git clone https://github.com/github/hub.git
cd hub  
script/build  
mv ./hub ~/bin # ← move to somewhere in your PATH

here is the magic…

hub pull-request -b <target_username>/<target_repo>:<target_branch_name>

done.

ps: if you pass -o will open the pr in your default browser