configitems: register the 'extdata' section

mercurial/configitems.py

@@ -420,6 +420,10 b" coreconfigitem('extensions', '.*',"
     default=None,
     generic=True,
 )
+coreconfigitem('extdata', '.*',
+    default=None,
+    generic=True,
+)
 coreconfigitem('format', 'aggressivemergedeltas',
     default=False,
 )