The inertia of history is why I have a 'mozilla-hg' directory with a 'gecko-dev' subdirectory in it that is a Git repository. And there's more inertia of history than you might expect, because the reason it's 'mozilla-hg' and not 'mozilla' is that I once pulled Firefox source with CVS, back in the days, until they moved over to Mercurial. So my new Mercurial checkout got put in 'mozilla-hg'.