check: Only track data blobs when unused blobs should be reported

This improves the memory usage of check a lot as it now only has to
track tree blobs when run using the default parameters.

cmd/restic/cmd_check.go

@@ -193,7 +193,7 @@ func runCheck(opts CheckOptions, gopts GlobalOptions, args []string) error {
 		}
 	}
 
-	chkr := checker.New(repo)
+	chkr := checker.New(repo, opts.CheckUnused)
 
 	Verbosef("load indexes\n")
 	hints, errs := chkr.LoadIndex(gopts.ctx)