Hello!

Someone on #guix reported a failure to build Guile-Git from Guix 0.13.0
because the old repo at gitlab.com has disappeared.

For tarballs, we have content addressed mirrors, in particular the /file
URL of ‘guix publish’.  However, that’s only for regular files, not for
directories like Git checkouts.

For directories (and store items in general), we have the /nar URLs
though (normally used for substitutes).  This patch uses /nar URLs as a
fallback mirror (it’s content-addressed, even though the hash in the URL
is not directly the content hash) for Git clones that fail.

It’s rough on the edges (no TLS, no compression), but it shows that it’s
a viable solution.  It would take some thought to avoid duplicating it
between git, hg, etc.

Thoughts?

Ludo’.