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

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.

--- a/mercurial/ui.py	Sat Jun 17 12:54:04 2017 +0200
+++ b/mercurial/ui.py	Sat Jun 17 12:54:45 2017 +0200
@@ -649,7 +649,7 @@
             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 @@
         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):