The main advantage of using an interface over an abstract class in this instance is that an interface is entirely transparent: This is more of an issue where you don't have access to the source of the class you're inheriting from.
However, this transparency allows you to produce unit tests of a known scope: If you test a class that accepts an interface as a parameter (using the dependency injection method), you know you're testing the class with a known quantity; the testing implementation of the interface will only contain your testing code.
Similarly, when testing your repository, you know you're testing just your code in the repository. This helps to limit the number of possible variables/interactions in the test.
