config: use '_config' within 'configbytes'

This will prevent bugs from using None as the sentinel value (eg:
'ui.interactive')

mercurial/ui.py

@@ -634,8 +634,8 b' class ui(object):'
         ConfigError: foo.invalid is not a byte quantity ('somevalue')
         """
 
-        value = self.config(section, name, default, untrusted)
-        if value is None:
+        value = self._config(section, name, default, untrusted)
+        if value is _unset:
             if default is _unset:
                 default = 0
             value = default