Diff class not mockable

The Diff class is not very mockable due to its dependency on a private Repository field that is only set from an internal constructor. I encountered this when attempting to mock a repository and its related object graph and then invoking Commands.Stage with that mocked repository. It resulted in a null ref because the Diff.repo field wasn't set.

Commands.Stage ends up calling the internal Diff.Compare method:
https://github.com/libgit2/libgit2sharp/blob/3d7276275e8ea9dce324e437760c31e69b5ec10c/LibGit2Sharp/Commands/Stage.cs#L77

Compare ends up passing the Diff.repo field to WorkdirToIndex:
https://github.com/libgit2/libgit2sharp/blob/3d7276275e8ea9dce324e437760c31e69b5ec10c/LibGit2Sharp/Diff.cs#L472

Which then blows up because repo is null:
https://github.com/libgit2/libgit2sharp/blob/3d7276275e8ea9dce324e437760c31e69b5ec10c/LibGit2Sharp/Diff.cs#L506

The repo field is null because it only gets set by an internal constructor:
https://github.com/libgit2/libgit2sharp/blob/3d7276275e8ea9dce324e437760c31e69b5ec10c/LibGit2Sharp/Diff.cs#L90

This prevents a good mocking solution because the internal constructor can't be called without reflection.

Start with LibGit2Sharp/Diff.cs, especially the internal constructor, Compare, and WorkdirToIndex, then trace the call from LibGit2Sharp/Commands/Stage.cs. Reproduce the mocked-repository Commands.Stage scenario described in the issue. Done means the path no longer depends on reflection to construct a usable Diff and does not fail because its repository is null.