demandimport: omit default value of "level" at construction of _demandmod

This makes construction of _demandmod require explicit level value,
and helps to avoid issues like issue5208 in the future.

mercurial/demandimport.py

@@ -64,8 +64,12 b' def _hgextimport(importfunc, name, globa'
         return importfunc(hgextname, globals, *args, **kwargs)
 
 class _demandmod(object):
-    """module demand-loader and proxy"""
-    def __init__(self, name, globals, locals, level=level):
+    """module demand-loader and proxy
+
+    Specify 1 as 'level' argument at construction, to import module
+    relatively.
+    """
+    def __init__(self, name, globals, locals, level):
         if '.' in name:
             head, rest = name.split('.', 1)
             after = [rest]