configitems: support callable as a default value

Yuya pointed out that using mutable value as the default could be problematic.
To work around this we now support callable object as default value. This
allows for creating new mutable objects on demand when needed.

mercurial/configitems.py

@@ -60,7 +60,7 b" coreconfigitem('patch', 'fuzz',"
     default=2,
 )
 coreconfigitem('ui', 'clonebundleprefers',
-    default=[],
+    default=list,
 )
 coreconfigitem('ui', 'interactive',
     default=None,

mercurial/ui.py

@@ -455,6 +455,8 b' class ui(object):'
             if default is _unset:
                 if item is None:
                     value = default
+                elif callable(item.default):
+                    value = item.default()
                 else:
                     value = item.default
             elif item is not None: