further fix traceback on invalid .hgignore patterns

Please apply this little fix (matchfn return None if no patterns are
specified).

mercurial/util.py

@@ -246,6 +246,8 b' def _matcher(canonroot, cwd, names, inc,'
 
     def matchfn(pats, tail):
         """build a matching function from a set of patterns"""
+        if not pats:
+            return
         matches = []
         for k, p in pats:
             try: