configitems: register 'ui.clonebundleprefers' as example for 'configlist'

This exercise the default value handling in 'configlist'.

mercurial/configitems.py

@@ -39,6 +39,9 b' def coreconfigitem(*args, **kwargs):'
 coreconfigitem('patch', 'fuzz',
     default=2,
 )
+coreconfigitem('ui', 'clonebundleprefers',
+    default=[],
+)
 coreconfigitem('ui', 'quiet',
     default=False,
 )

mercurial/exchange.py

@@ -1980,7 +1980,7 b' class clonebundleentry(object):'
         return self._cmp(other) != 0
 
 def sortclonebundleentries(ui, entries):
-    prefers = ui.configlist('ui', 'clonebundleprefers', default=[])
+    prefers = ui.configlist('ui', 'clonebundleprefers')
     if not prefers:
         return list(entries)