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.