Update repository size on cron gc task (#15177)

git gc cron could change the size of the repository therefore we should update the
size of the repo stored in our database.

Also significantly improve the efficiency of counting lfs associated with the
repository

models/repo.go

@@ -740,15 +740,12 @@ func (repo *Repository) updateSize(e Engine) error {
 		return fmt.Errorf("updateSize: %v", err)
 	}
 
-	objs, err := repo.GetLFSMetaObjects(-1, 0)
+	lfsSize, err := e.Where("repository_id = ?", repo.ID).SumInt(new(LFSMetaObject), "size")
 	if err != nil {
 		return fmt.Errorf("updateSize: GetLFSMetaObjects: %v", err)
 	}
-	for _, obj := range objs {
-		size += obj.Size
-	}
 
-	repo.Size = size
+	repo.Size = size + lfsSize
 	_, err = e.ID(repo.ID).Cols("size").Update(repo)
 	return err
 }