Replaced cmdutil.command with registrar.command

Created originally on Bitbucket by dsparkplug (Dave Sparks)

The cmdutil.command is no longer available as from hg 4.7.

See https://www.mercurial-scm.org/repo/hg/rev/fb0de0bcd297

Original Bitbucket Pull Request is from fork dsparkplug/win32lfn that has not been imported