feat: add parameter to checkout to specify a remote tracking branch

[armcknight]

I needed this because I have some automation set up for managing forks locally. The way I like to do this is by having two remotes, upstream and fork, and i track each one's default branch separately locally in branches named fork_main and upstream_main, but since those aren't the remote names of those refs, I need to set the tracking branch manually when running git checkout -b