[bug#37443] [PATCH] scripts: pull: Add options for generation management

[Konrad Hinsen]

Hi everyone,

let me add an explanation as for why I propose these additions
to guix pull.

I have been working on a tutorial on using Guix for reproducible
research, which will be part of a MOOC on reproducible research.  In
that tutorial, I focus on using "guix environment" with manifest files
for defining computational environemnts. Next, I introduce "guix
describe" and "guix pull -C" to document the version of Guix that has
been used.

At the very end, I use a roll-back to undo the effect of "guix pull -C".
At that point, I had to introduce "guix package" that I never used
before, and also the concept of profiles that I hadn't needed so far
either. All that just for a roll-back!

It is of course nice that internally, the same profile management code
is used for Guix and for installed packages, but I don't think that
users should have to know about that. Moreover, users shouldn't have to
memorize the path of the Guix profile either. With the patch I sent,
Guix generations are entirely managed by "guix pull".

Cheers,
  Konrad