* guix/scripts/refresh.scm (%options): Register 'target-version' long version. (update-specification->update-spec): Add a fallback-version argument. (options->update-specs): Honor target-version option. * tests/guix-refresh.sh: Test it. * doc/guix.texi (Invoking guix refresh): Document it. Reviewed-by: Ludovic Courtès <ludo@gnu.org>