Hello,

Here's a small patch to (guix git) so that cloning/fetching from ssh
authenticated repositories is supported using ssh agent.

I tested:

* guix pull --url=git@gitlab.com:mothacehe/private.git
* guix pull with the following channel configuration

--8<---------------cut here---------------start------------->8---
(cons* (channel
        (name 'gitlab)
        (url "git@gitlab.com:mothacehe/test-channel.git"))
       %default-channels)
--8<---------------cut here---------------end--------------->8---

This works fine, but we still need to see how it works for Cuirass
inputs and (guix git-download) module.

Mathieu