Fix error when calculate the repository size (#22392)

Fix #22386 

`GetDirectorySize` moved as `getDirectorySize` because it becomes a
special function which should not be put in `util`.

Co-authored-by: Jason Song <i@wolfogre.com>

models/fixtures/repository.yml

@@ -24,7 +24,7 @@
   fork_id: 0
   is_template: false
   template_id: 0
-  size: 0
+  size: 6708
   is_fsck_enabled: true
   close_issues_via_commit_in_any_branch: false
 

models/repo/update.go

@@ -184,7 +184,7 @@ func ChangeRepositoryName(doer *user_model.User, repo *Repository, newRepoName s
 	return committer.Commit()
 }
 
-// UpdateRepoSize updates the repository size, calculating it using util.GetDirectorySize
+// UpdateRepoSize updates the repository size, calculating it using getDirectorySize
 func UpdateRepoSize(ctx context.Context, repoID, size int64) error {
 	_, err := db.GetEngine(ctx).ID(repoID).Cols("size").NoAutoTime().Update(&Repository{
 		Size: size,