R1:cb5a72b5e6d1

Fix call to non-existing getDisplayName() in DiffusionRepositoryClusterEngine.php

Summary:
`DiffusionRepositoryClusterEngine` calls `$this->getDisplayName()` which is not defined in this class or its parent class.
Looking at the variable use in the companying error message, getting the name of the `$repository` was intended instead.

Test Plan: Carefully read the code and check available methods in its parent classes.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers…