Introduce StatusOptions.IncludeUnaltered

Test can be improved, what else needs done?
Or, even better maybe, **[directly assert](https://github.com/libgit2/libgit2sharp/blob/7a317883c304b12e4d1a742e8a61ddb751985f8d/LibGit2Sharp.Tests/BranchFixture.cs#L287)** the expected filenames?