dirstate-v2: highlight a bug when Python-packed but used in `rhg`

The Python packer creates unsorted entries in the edge case that a file starts with the same name as a sibling folder.

This bug has no effect on the Python hg status since Python ignores directories. rhg assumes that all on-disk entries are sorted (which is a property of the format) including folder, hence the issue highlighted.

This is also technically broken in Rust-augmented hg status, but it makes setting up the test more complex than necessary, since it requires the packing to be Python only (which it isn't if you have Rust extensions).

Fix is in the next commit.