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

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

The default value for date (None) is still enforced in this method if no other
default were passed.

mercurial/ui.py

@@ -649,7 +649,7 b' class ui(object):'
             return []
         return v
 
-    def configdate(self, section, name, default=None, untrusted=False):
+    def configdate(self, section, name, default=_unset, untrusted=False):
         """parse a configuration element as a tuple of ints
 
         >>> u = ui(); s = 'foo'
@@ -660,6 +660,8 b' class ui(object):'
         if self.config(section, name, default, untrusted):
             return self.configwith(util.parsedate, section, name, default,
                                    'date', untrusted)
+        if default is _unset:
+            return None
         return default
 
     def hasconfig(self, section, name, untrusted=False):