This allows for Python code to call parse/pack_dirstate transparently.
These bindings are heavy given the relatively simple task, as they are bound
to implementation details of both the C and Python code. They will be slimmed
down in future patches and eventually completely removed once more of the
dirstate code has been refactored/rewritten in Rust.
Both functions emulate the mutate-on-loop style of the Python and C
implementations by looping over changed items in the compatibility layer,
instead of at the core functions.