not update updated uinx for `git gc` (#15637)

fix #15634

Signed-off-by: a1012112796 <1012112796@qq.com>

models/repo.go

@@ -747,7 +747,7 @@ func (repo *Repository) updateSize(e Engine) error {
 	}
 
 	repo.Size = size + lfsSize
-	_, err = e.ID(repo.ID).Cols("size").Update(repo)
+	_, err = e.ID(repo.ID).Cols("size").NoAutoTime().Update(repo)
 	return err
 }