Add the git commit id to the generation information

[Roel Janssen <roel@gnu.org>]

Dear Guix,

I have a feature request.

When I run 'guix pull', install a package, run 'guix pull' again (on
another day), and install another package, I'd get two generations:

> Generation 181  May 22 2017 11:47:37
>  + haunt        0.2.1   out     /gnu/store/x2bkpzrdbaxavl29r2xg0fip1jjici9q-haunt-0.2.1
>
> Generation 182  May 24 2017 09:52:06    (current)
>  + guile-commonmark     0.1     out     /gnu/store/k8vq65czfb8k4hvvjvsca34scd9xsxik-guile-commonmark-0.1

Now, if I'd like to go to the Guix packages source code of generation
181, I can only guess at which actual state of the Guix repository this
was, because this timestamp is the timestamp of creating the generation,
not the timestamp of the Guix package state.  So, I'd like to propose to
add the commit id to the generation like so:

> Generation 181  May 22 2017 11:47:37    (ae548434337cddf9677a4cd52b9370810b2cc9b6)
>  + haunt        0.2.1   out     /gnu/store/x2bkpzrdbaxavl29r2xg0fip1jjici9q-haunt-0.2.1
>
> Generation 182  May 24 2017 09:52:06    (current)
>  + guile-commonmark     0.1     out     /gnu/store/k8vq65czfb8k4hvvjvsca34scd9xsxik-guile-commonmark-0.1

So, 'current' is the one that is installed at that time.  Any other
should be traceable through the git commit ID.

This would probably require us to have the repository information
available when building the tarball that will be downloaded by 'guix
pull'.  I think this feature is very important because it makes the
generations traceable to the Guix source code, which is needed for
reproducing the same generation.

Is this something we could make?  And can we live with the implicit
requirement to have the git repository information available when
building the source tarballs used by 'guix pull'?

Kind regards,
Roel Janssen