Parse ref names that have slashes

Signed-off-by: Stefan Buller <stefan.buller@gmail.com>
Fix implementation of valid_restore_path

The former implementation simply made sure that there is at least one slash
in the target.

Since we now allow slashes in branchnames, this is not enough, so we actually
check for the type of the vfs-Node and reject BranchList and BranchDir entries.

Signed-off-by: Tim Riemenschneider <git@tim-riemenschneider.de>