ls: add missing read lock

As `ls` reads data from the repository, it must acquire a read lock
unless `--no-lock` was specified. The old behavior is equivalent to `ls
--no-lock`.

cmd/restic/cmd_ls.go

@@ -309,10 +309,11 @@ func runLs(ctx context.Context, opts LsOptions, gopts GlobalOptions, args []stri
 		return false
 	}
 
-	repo, err := OpenRepository(ctx, gopts)
+	ctx, repo, unlock, err := openWithReadLock(ctx, gopts, gopts.NoLock)
 	if err != nil {
 		return err
 	}
+	defer unlock()
 
 	snapshotLister, err := restic.MemorizeList(ctx, repo, restic.SnapshotFile)
 	if err != nil {