Bust that cache

@bkeepers - I'm having some trouble finding a good way to test this. I would expect `Rugged::Tree.diff` to be called twice ([here](https://github.com/github/linguist/pull/1588/files#diff-b4edef36c2cef485a8746b3196a07e62L136) and [here](https://github.com/github/linguist/pull/1588/files#diff-b4edef36c2cef485a8746b3196a07e62R139)) but that doesn't seem to be the case :confused: