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.

Bắt đầu với LibGit2Sharp/Diff.cs, đặc biệt là constructor nội bộ, Compare và WorkdirToIndex, sau đó lần theo lời gọi từ LibGit2Sharp/Commands/Stage.cs. Tái hiện kịch bản Commands.Stage với repository giả lập được mô tả trong issue. Được xem là hoàn tất khi đường dẫn này không còn phụ thuộc vào reflection để tạo một Diff có thể sử dụng và không thất bại vì repository của nó là null.