walker: extend TreeRewriter to support snapshot repairing

This adds support for caching already rewritten trees, handling of load
errors and disabling the check that the serialization doesn't lead to
data loss.

cmd/restic/cmd_rewrite.go

@@ -95,6 +95,7 @@ func rewriteSnapshot(ctx context.Context, repo *repository.Repository, sn *resti
 			Verbosef(fmt.Sprintf("excluding %s\n", path))
 			return nil
 		},
+		DisableNodeCache: true,
 	})
 
 	return filterAndReplaceSnapshot(ctx, repo, sn,