Use . as separator for filename when streaming to restic

Restic 0.9.3 fails when --stdin-filename contains slashes:

repository 954a4e61 opened successfully, password is correct
error: file does not exist
Fatal: unable to save snapshot: snapshot is empty

repository_restic.go

@@ -191,7 +191,7 @@ func (r *resticRepository) BackupStream(ctx context.Context, backup Backup, ds D
 	// Try to do the obvious thing with naming.
 	name := ds.Name
 	if len(ds.Atoms) == 1 {
-		name = fmt.Sprintf("%s/%s", ds.Name, ds.Atoms[0].Name)
+		name = fmt.Sprintf("%s.%s", ds.Name, ds.Atoms[0].Name)
 	}
 	return r.shell.Run(ctx, fmt.Sprintf(
 		"%s backup --stdin --stdin-filename %s",