blackbox: use a human readable version of the default

Now that the default value is also converted we can use a human readable version
for it. This will be useful if we start to automatically display the default
config value in various place.

hgext/blackbox.py

@@ -62,7 +62,7 b' configtable = {}'
 configitem = registrar.configitem(configtable)
 
 configitem('blackbox', 'maxsize',
-    default=1048576,
+    default='1 MB',
 )
 
 lastui = None