repository: Add tests fetch `FetchRemote()` with diverging refs

While we already have a bunch of tests, including some for diverging refs, there are still some constellations we don't fetch. For one, we don't have tests that verify that we indeed perform partial reference updates. And second, we don't have tests that verify that we fetch tags even though branches have diverged.

Add these tests to improve test coverage and prepare for truly-atomic fetches in FetchRemote().

Part of #3780 (closed).