guix: update-cached-checkout: Fix fetching remote for branches, commits and tags.
Follow up of 66463356ce5868d3551ea7014acb34543972a5d8.
Because of the mentioned change, update-cached-checkout doesn't
update the checkout for branches, commits or tags.
This means `guix pull` with %default-channels no longer pulls
newer versions. `forward-update-check` no longer fetches the
commit to check relations for, leading to an error that
reference is not available.
* guix/git.scm (update-cached-checkout): Fetch remote even if symref-list is
empty.
Change-Id: Ia6bb1c669065cf19a6dd16c2a403e8590bc07613