rust: error for WdirUnsupported with cpython conversion as exception This introduces WorkingDirectoryUnsupported as an enum variant of GraphError in the core and converts it to the expected `mercurial.error.WdirUnsupported`.

rust: working directory revision number constant This introduces the constant, but does not use it anywhere yet.

py3: fix test-update-atomic.t The .replace('o', '') hack on oct() return value is required because: % python -c 'print(oct(10))' 012 % python3 -c 'print(oct(10))' 0o12 Differential Revision: https://phab.mercurial-scm.org/D5684