Hello,

When "guix pull" is run before the CI server is done baking substitutes,
the user may need to build some derivations, which can be long on
low-end hardware.

This patch adds an option to "guix pull" so that it updates to the
latest commit with available substitutes.

This work is still in progress, but I'd like to gather some impressions
on that before going further.

Thanks,

Mathieu