configitems: get default values from the central registry when available

configitems: get default values from the central registry when available

We do not have any registered config yet, but we are now ready to use them.

For now we ignore this feature for config access with "alternates". On the long
run, we expect alternates to be handled as "aliases" by the config item
themself.

--- a/mercurial/ui.py	Sat Jun 17 18:43:27 2017 +0200
+++ b/mercurial/ui.py	Sat Jun 17 12:15:28 2017 +0200
@@ -439,11 +439,17 @@
         return self._data(untrusted).source(section, name)
 
     def config(self, section, name, default=_unset, untrusted=False):
-        if default is _unset:
-            default = None
         if isinstance(name, list):
             alternates = name
+            # let us ignore the config items in the alternates case for now
+            if default is _unset:
+                default = None
         else:
+            if default is _unset:
+                default = None
+                item = self._knownconfig.get(section, {}).get(name)
+                if item is not None:
+                    default = item.default
             alternates = [name]
 
         for n in alternates: