As discussed on help-guix list (somewhere near <http://lists.gnu.org/archive/html/help-guix/2016-06/msg00129.html>), this is to make it possible to use "M-x guix" by default (currently emacs gives an error about missing 'magit-popup' unless a user explicitly installs magit).
[PATCH 1/2] gnu: Add emacs-magit-popup. [PATCH 2/2] gnu: guix: Add emacs-magit-popup to propagated-inputs.
