bug#22883: Trustable "guix pull"

[Christopher Allan Webber]

Right now, when a user does a "guix pull", that pulls down the latest
repository of code from git, which is kept in a tarball.  Once you
receive the latest code, this has some checks: what's the hash of each
package, etc.

Unfortunately, it's delivered over http:

  (define %snapshot-url
    ;; "http://hydra.gnu.org/job/guix/master/tarball/latest/download";
    "http://git.savannah.gnu.org/cgit/guix.git/snapshot/master.tar.gz";
    )

At minimum we should deliver this over HTTPS, ideally with a single
certificate that is trusted by the user, so the user can't be easily
MITM'ed.

On top of that, even if you run from git proper what there isn't a test
about is: can you trust those latest commits?  Git doesn't really check,
at least by default.

  https://mikegerwitz.com/papers/git-horror-story

How about this: anyone with commit access should use "signed off by" and
gpg signatures combined.  We should keep some list of guix committers'
gpg keys.  No commit should be pushed to guix without a gpg signature.
At this point, at least, there is some possibility of auditing things.

Perhaps before a master.tar.gz is made, there can be some integrity
check of the commits matching the current set of "trusted" keys?