convert MemorizeList to be repository based

Ideally, code that uses a repository shouldn't directly interact with
the underlying backend. Thus, move MemorizeList one layer up.

cmd/restic/cmd_dump.go

@@ -147,7 +147,7 @@ func runDump(ctx context.Context, opts DumpOptions, gopts GlobalOptions, args []
 		Hosts: opts.Hosts,
 		Paths: opts.Paths,
 		Tags:  opts.Tags,
-	}).FindLatest(ctx, repo.Backend(), repo, snapshotIDString)
+	}).FindLatest(ctx, repo, repo, snapshotIDString)
 	if err != nil {
 		return errors.Fatalf("failed to find snapshot: %v", err)
 	}