Also add missing installPhase hooks, and clarify license.
I'm assuming GPL-2.0-only, because it doesn't seem to be specified, and
the helper links with git, which is -only.
git-cinnabar calls into Mercurial as a library, so we make use the
same version of Python as Mercurial.
Support for Python 3 is an experimental feature in git-cinnabar, but
we unconditionally use it here because:
* Mercurial in Nixpkgs only supports Python 3.
* git-cinnabar touches a network, and for that purpose it's good to
use a version of Python that is going to get security updates.
