match: explicitly tests for None

Changeset 6168d4b93634 removed the mutable default value, but did not explicitly
tested for None. Such implicit testing can introduce semantic and performance
issue. We move to an explicit testing for None as recommended by PEP8:

https://www.python.org/dev/peps/pep-0008/#programming-recommendations

mercurial/match.py

@@ -117,8 +117,10 b' class match(object):'
                               the same directory
         '<something>' - a pattern of the specified default type
         """
-        include = include or []
-        exclude = exclude or []
+        if include is None:
+            include = []
+        if exclude is None:
+            exclude = []
 
         self._root = root
         self._cwd = cwd