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

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

The default value for bool (False) is still enforced in this method if no other
default were passed.

mercurial/ui.py

@@ -505,7 +505,7 b' class ui(object):'
                 v = os.path.join(base, os.path.expanduser(v))
         return v
 
-    def configbool(self, section, name, default=False, untrusted=False):
+    def configbool(self, section, name, default=_unset, untrusted=False):
         """parse a configuration element as a boolean
 
         >>> u = ui(); s = 'foo'
@@ -526,8 +526,10 b' class ui(object):'
         ConfigError: foo.invalid is not a boolean ('somevalue')
         """
 
-        v = self.config(section, name, None, untrusted)
+        v = self.config(section, name, default, untrusted=untrusted)
         if v is None:
+            if default is _unset:
+                return False
             return default
         if isinstance(v, bool):
             return v