rust-pyo3-index: _index_headrevs This one demonstrates that why the `with_index_read` and similar helpers are useful and was actually the main motivation for doing them: if we kept the borrow used to grab the index before updating the caches, there would be a panic when calling `borrow_mut`. This was confirmed with an earlier version by the Python test. There are perhaps some internal API clarifications to be made, as the method updating the cache does a seemingly useless return), but we are keeping it as it was in `hg-cpython`.