Fix issues on repositories with non-Unicode encodings

[muravvv]

Fix two issues, which prevents aider from using on repositories with single-byte encodings:

1. Fix inability to use --llm-history-file option when non-Unicode encoding is set by --encoding. Before this fix any action in aider with this options results in following error:

Traceback (most recent call last):
  File "C:\Users\Vitya\AppData\Roaming\uv\tools\aider-chat\Lib\site-packages\aider\coders\base_coder.py", line 1454, in
send_message
    yield from self.send(messages, functions=self.functions)
  File "C:\Users\Vitya\AppData\Roaming\uv\tools\aider-chat\Lib\site-packages\aider\coders\base_coder.py", line 1788, in
send
    self.io.log_llm_history("TO LLM", format_messages(messages))
  File "C:\Users\Vitya\AppData\Roaming\uv\tools\aider-chat\Lib\site-packages\aider\io.py", line 754, in log_llm_history
    log_file.write(content + "\n")
  File "C:\Program Files\Python312\Lib\encodings\cp1251.py", line 19, in encode
    return codecs.charmap_encode(input,self.errors,encoding_table)[0]
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
UnicodeEncodeError: 'charmap' codec can't encode character '\u22ee' in position 8769: character maps to <undefined>

The symbol ⋮ (\u22ee) is used by aider to mark skipped lines in repo map. So to fix this bug I have made utf-8 encoding to be always used for .aider.llm.history log regardless of --encoding option.

2. Fix impossibility to generating commit message if changed lines (or diff context lines) contain non-UTF-8 symbols (non-ASCII symbols in single-byte encoding). I have seen this bug with Ollama models, but most likely the problem arises with most of other models. In this case error message looks like:

litellm.APIConnectionError: 'utf-8' codec can't encode characters in position 3618-3626: surrogates not allowed
Traceback (most recent call last):
  File "C:\Users\user\AppData\Roaming\uv\tools\aider-chat\Lib\site-packages\litellm\main.py", line 2937, in completion
    generator = ollama_chat.get_ollama_response(
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\user\AppData\Roaming\uv\tools\aider-chat\Lib\site-packages\litellm\llms\ollama_chat.py", line 313, in get_ollama_response
    response = sync_client.post(
               ^^^^^^^^^^^^^^^^^
  File "C:\Users\user\AppData\Roaming\uv\tools\aider-chat\Lib\site-packages\litellm\llms\custom_httpx\http_handler.py", line 578, in post
    raise e
  File "C:\Users\user\AppData\Roaming\uv\tools\aider-chat\Lib\site-packages\litellm\llms\custom_httpx\http_handler.py", line 554, in post
    req = self.client.build_request(
          ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\user\AppData\Roaming\uv\tools\aider-chat\Lib\site-packages\httpx\_client.py", line 378, in build_request
    return Request(
           ^^^^^^^^
  File "C:\Users\user\AppData\Roaming\uv\tools\aider-chat\Lib\site-packages\httpx\_models.py", line 408, in __init__
    headers, stream = encode_request(
                      ^^^^^^^^^^^^^^^
  File "C:\Users\user\AppData\Roaming\uv\tools\aider-chat\Lib\site-packages\httpx\_content.py", line 216, in encode_request
    return encode_json(json)
           ^^^^^^^^^^^^^^^^^
  File "C:\Users\user\AppData\Roaming\uv\tools\aider-chat\Lib\site-packages\httpx\_content.py", line 179, in encode_json
    ).encode("utf-8")
      ^^^^^^^^^^^^^^^
UnicodeEncodeError: 'utf-8' codec can't encode characters in position 3618-3626: surrogates not allowed

Retrying in 0.2 seconds...

The problem was arised from using wrong encoding settings in parsing of git diff command output: it use default encoding for filenames (which is utf-8 on most systems) instead of encoding set by --encoding option, In addtion to wrong encoding, PythonGit also have used surrogateescape error handling mode which produce wrong Unicode surrogates, that cannot be converted to UTF-8 when sending to models.

[CLAassistant]

All committers have signed the CLA.

[paul-gauthier]

Thanks!