orphan commit creation

gistfile1.cs

var blob = repo.ObjectDatabase.CreateBlob("index.txt");
var treeDefinition = new TreeDefinition();
treeDefinition.Add("index.txt", blob, Mode.NonExecutableFile);
var tree = repo.ObjectDatabase.CreateTree(treeDefinition);

var commit = repo.ObjectDatabase.CreateCommit("Hello World",
    new Signature("name", "mail", DateTimeOffset.Now),
    new Signature("name", "mail", DateTimeOffset.Now),
    tree, new Commit[0]);


repo.Branches.Add("index", commit);