rust-dirstate: create dirstate submodule in hg-cpython
This module will soon hold multiple files, this change is to make the
review process easier.
Differential Revision: https://phab.mercurial-scm.org/D6626
Greg Ward, author of the original bfiles extension
Na'Tosha Bard of Unity Technologies
Fog Creek Software
Special thanks to the University of Toronto and the UCOSP program