This was a HashMap constructed on demand and then cached in the DirstateMap
struct to avoid reconstructing at the next access. However the only use is
in Python bindings converting it to a PyDict. That method in turn is wrapped
in a @cachedproperty in Python code.
This was two redudant layers of caching. This changeset removes the Rust-level
one to keep the Python dict cache, and have bindings create a PyDict by
iterating.