Use buffersize to reduce database connection when iterate (#2724)

* use buffersize to reduce database connection when iterate

* fix typo

* add default value on app.ini comment

models/migrations/v19.go

@@ -42,7 +42,7 @@ func generateAndMigrateGitHooks(x *xorm.Engine) (err error) {
 		}
 	)
 
-	return x.Where("id > 0").Iterate(new(Repository),
+	return x.Where("id > 0").BufferSize(setting.IterateBufferSize).Iterate(new(Repository),
 		func(idx int, bean interface{}) error {
 			repo := bean.(*Repository)
 			user := new(User)