Use 'if not' instead of 'if ... is False' for no_fuzzy_matching

Matches the behavior elsewhere in the same function

Fixes #698

babel/messages/catalog.py

@@ -807,7 +807,7 @@ def _merge(message, oldkey, newkey):
                 if key in messages:
                     _merge(message, key, key)
                 else:
-                    if no_fuzzy_matching is False:
+                    if not no_fuzzy_matching:
                         # do some fuzzy matching with difflib
                         if isinstance(key, tuple):
                             matchkey = key[0]  # just the msgid, no context