Adds a shorthand to checkout a remote branch with the prefix "others".

Given a remote branch <b> it will execute
git checkout --track -b others/<b> origin/<b>

This is often useful when we want to checkout someone else branch but we want to clearly distinguish it from the personal branches.

Personally I use it a lot when reviewing PRs. 
I want to get a certain branch to review the details but I don't want to mess up my space.