registrar: move cmdutil.command to registrar module (API) cmdutil.command wasn't a member of the registrar framework only for a historical reason. Let's make that happen. This patch keeps cmdutil.command as an alias for extension compatibility.

To run the tests, do:

cd tests/
python run-tests.py

See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.