[Currently](https://github.com/libgit2/libgit2sharp/blob/reflog-refactor/LibGit2Sharp.Tests/ReflogFixture.cs#L34-L41), trying to read a nonexistent reflog throws `InvalidSpecificationException`. This means there's no such thing as an "unborn" `ReflogCollection`, so we can't do `repo.Refs.Log("refs/heads/newbranch").Ensure()`.