I would like to be able to mirror a client's Git repository, create a new branch, and then pull changes from the Git repository without pushing my unfinished/proprietary/scratch/experimental branch.

I have also, from time to time, found myself wanting to clone just a single branch from Git, exclude some branches from Git, or pull everything without pushing anything. All of this is possible in Plastic sync view with Plastic repos, so I thought it would be better to suggest integrating the two instead of suggesting to add these features independently to git-sync.