using (var repo = new Repository("path/to/your/repo"))
{
    // Write content to file system
    var content = "Commit this!";
    File.WriteAllText(Path.Combine(repo.Info.WorkingDirectory, "fileToCommit.txt"), content);

    // Stage the file
    repo.Index.Add("fileToCommit.txt");
    repo.Index.Write();

    // Create the committer's signature and commit
    Signature author = new Signature("James", "@jugglingnutcase", DateTime.Now);
    Signature committer = author;

    // Commit to the repository
    Commit commit = repo.Commit("Here's a commit i made!", author, committer);
}

using (Repository repo = new Repository("path-to-bare-repo"))
{
    string content = "Hello commit!";

    // Create a blob from the content stream
    byte[] contentBytes = System.Text.Encoding.UTF8.GetBytes(content);
    MemoryStream ms = new MemoryStream(contentBytes);
    Blob newBlob = repo.ObjectDatabase.CreateBlob(ms);

    // Put the blob in a tree
    TreeDefinition td = new TreeDefinition();
    td.Add("filePath.txt", newBlob, Mode.NonExecutableFile);
    Tree tree = repo.ObjectDatabase.CreateTree(td);

    // Committer and author
    Signature committer = new Signature("James", "@jugglingnutcase", DateTime.Now);
    Signature author = committer;

    // Create binary stream from the text
    Commit commit = repo.ObjectDatabase.CreateCommit(
        author,
        committer,
        "i'm a commit message :)",
        tree,
        repo.Commits,
        prettifyMessage: false);

    // Update the HEAD reference to point to the latest commit
    repo.Refs.UpdateTarget(repo.Refs.Head, commit.Id);
}