encoding.upper: factor out fallback code

This will be used as the fallback function on Windows.

mercurial/encoding.py

@@ -338,7 +338,9 b' def upper(s):'
     try:
         return asciiupper(s)
     except UnicodeDecodeError:
-        pass
+        return upperfallback(s)
+
+def upperfallback(s):
     try:
         if isinstance(s, localstr):
             u = s._utf8.decode("utf-8")