encoding: protect against non-ascii default encoding stable

encoding: protect against non-ascii default encoding

If the default python encoding was changed from ascii, the attempt to
encode as ascii before lower() could throw a UnicodeEncodeError.
Catch UnicodeError instead to prevent an unhandled exception.

--- a/mercurial/encoding.py	Mon Apr 23 14:32:59 2012 +0200
+++ b/mercurial/encoding.py	Sun Apr 22 21:27:52 2012 -0400
@@ -169,7 +169,7 @@
     "best-effort encoding-aware case-folding of local string s"
     try:
         return s.encode('ascii').lower()
-    except UnicodeDecodeError:
+    except UnicodeError:
         pass
     try:
         if isinstance(s, localstr):