configitems: fix registration for 'blackbox.track' config

Default mutable values could be problematic. Use a lambda that returns the
value instead.

Thanks to martin for catching this bug.

hgext/blackbox.py

@@ -71,7 +71,7 configitem('blackbox', 'logsource',
     default=False,
 )
 configitem('blackbox', 'track',
-    default=['*'],
+    default=lambda: ['*'],
 )
 
 lastui = None