further fix traceback on invalid .hgignore patterns

further fix traceback on invalid .hgignore patterns

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

--- a/mercurial/util.py	Wed Oct 26 16:10:36 2005 -0700
+++ b/mercurial/util.py	Wed Oct 26 16:10:56 2005 -0700
@@ -246,6 +246,8 @@
 
     def matchfn(pats, tail):
         """build a matching function from a set of patterns"""
+        if not pats:
+            return
         matches = []
         for k, p in pats:
             try: