Add a unit test leveraging the `SecureUsernamePasswordCredentials` type to perform a network operation against a private repository. For inspiration, something similar can be peeked at **[here](https://github.com/libgit2/libgit2sharp/blob/63f8d6d90f06f4578604b57502f2c6b8aabf4479/LibGit2Sharp.Tests/CloneFixture.cs#L199-L223)** /cc @ckuhn203