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.

Commencez par LibGit2Sharp/Diff.cs, en particulier le constructeur interne, Compare et WorkdirToIndex, puis suivez l’appel depuis LibGit2Sharp/Commands/Stage.cs. Reproduisez le scénario Commands.Stage avec un dépôt mocké décrit dans l’issue. C’est terminé lorsque le chemin ne dépend plus de la reflection pour construire un Diff utilisable et n’échoue pas parce que son dépôt est null.