Fix misinterpretation of the disabled flag

The old implementation was misinterpretating the disabled flag and
effectively applying memoization even when explicitly disabled.
The 'or' operator is a short-circuit one; namely, it evaluates the
second argument if and only if the first is False. Therefore the
following conditions caused unexpected side effects:

- memoize.disabled = True, key not yet memoized

  Having disabled the memoize function wrapper, the client expects
  that no memoization happens. Instead the execution enters the
  if clause and store the value into the 'memo' dictionary

- memoize.disabled = True, key memoized

  Having disabled the memoize function wrapper, the client expects
  that no memoization happens and the function will be evaluated
  anyway, whether or not its return value had already been stored in
  the 'memo' dictionary by a previous call. On the contrary, the last
  statement of wrapper() access the value stored by the last function
  execution.

This commit attempts to improve the function readability too.

thefuck/utils.py

@@ -23,11 +23,16 @@ def memoize(fn):
 
     @wraps(fn)
     def wrapper(*args, **kwargs):
-        key = pickle.dumps((args, kwargs))
-        if key not in memo or memoize.disabled:
-            memo[key] = fn(*args, **kwargs)
+        if not memoize.disabled:
+            key = pickle.dumps((args, kwargs))
+            if key not in memo:
+                memo[key] = fn(*args, **kwargs)
+            value = memo[key]
+        else:
+            # Memoize is disabled, call the function
+            value = fn(*args, **kwargs)
 
-        return memo[key]
+        return value
 
     return wrapper
 memoize.disabled = False