configitems: fix registration of extensions config

The new 'itemregister' class should be used when loading the extension config.
This was and oversight when itemregister was introduced.

mercurial/configitems.py

@@ -18,7 +18,7 b' from . import ('
 def loadconfigtable(ui, extname, configtable):
     """update config item known to the ui with the extension ones"""
     for section, items in configtable.items():
-        knownitems = ui._knownconfig.setdefault(section, {})
+        knownitems = ui._knownconfig.setdefault(section, itemregister())
         knownkeys = set(knownitems)
         newkeys = set(items)
         for key in sorted(knownkeys & newkeys):