aider: Fix variable name error in summarize method in history.py.

# Aider chat conversation:

USER:   File "/Users/gauthier/Projects/aider/aider/coders/base_coder.py", line 395, in run_loop
    return self.send_new_user_message(inp)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/gauthier/Projects/aider/aider/coders/base_coder.py", line 479, in send_new_user_message
    self.move_back_cur_messages(saved_message)
  File "/Users/gauthier/Projects/aider/aider/coders/base_coder.py", line 365, in move_back_cur_messages
    self.done_messages = self.summarizer.summarize(self.done_messages)
                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/gauthier/Projects/aider/aider/history.py", line 60, in summarize
    tail_tokens = sum(tokens for tokens, msg in sized[num:])
                                                      ^^^
NameError: name 'num' is not defined. Did you mean: 'sum'?

ASSISTANT: It seems like there's a typo in the code. The variable `num` is not defined in the current scope. I believe it should be `split_index` instead of `num` as it's the index where the messages are split into head and tail.

Here's the change that needs to be made in the `aider/history.py` file:

```python
aider/history.py
<<<<<<< ORIGINAL
        tail_tokens = sum(tokens for tokens, msg in sized[num:])
=======
        tail_tokens = sum(tokens for tokens, msg in sized[split_index:])
>>>>>>> UPDATED
```

Please confirm if this is the change you want to make.

aider/history.py

@@ -57,7 +57,7 @@ class ChatSummary:
 
         summary = self.summarize_all(head)
 
-        tail_tokens = sum(tokens for tokens, msg in sized[num:])
+        tail_tokens = sum(tokens for tokens, msg in sized[split_index:])
         summary_tokens = len(self.tokenizer.encode(json.dumps(summary)))
 
         result = summary + tail