$ git init /d/temp/rooted/path

string rootedPath = Repository.Init(@"D:\temp\rooted\path");
Console.WriteLine(rootedPath);

$ git add --all .

using (var repo = new Repository("path/to/your/repo"))
{
    Commands.Stage(repo, "*");
}

$ git checkout <branch>

using (var repo = new Repository("path/to/your/repo"))
{
    var branch = repo .Branches[branchName];
    Branch currentBranch = Commands.Checkout(repo , branch);
}