config: use the new '_unset' value for 'configint'

This should let 'configint' delegates all special processing of the default
config value to the main 'config' method.

mercurial/ui.py

@@ -574,7 +574,7 b' class ui(object):'
             raise error.ConfigError(_("%s.%s is not a valid %s ('%s')")
                                     % (section, name, desc, v))
 
-    def configint(self, section, name, default=None, untrusted=False):
+    def configint(self, section, name, default=_unset, untrusted=False):
         """parse a configuration element as an integer
 
         >>> u = ui(); s = 'foo'