Improve RemoteProgress parse return typing

[pick7]

Summary

• Broaden RemoteProgress._parse_progress_line() from None to Optional[object] so subclass overrides may return a value without violating the base type.
• Make the base implementation return None explicitly and keep new_message_handler() ignoring that return value.
• Add a mypy-covered regression test with an override that returns a non-None value.

Closes #1834

Checks

• .venv/bin/python -m pytest test/deprecation/test_remote_progress.py
• .venv/bin/python -m mypy git/util.py test/deprecation/test_remote_progress.py
• .venv/bin/python -m ruff check git/util.py test/deprecation/test_remote_progress.py
• .venv/bin/python -m ruff format --check git/util.py test/deprecation/test_remote_progress.py

AI agent disclosure

OpenAI Codex prepared this scoped code/test change and PR text. I reviewed the diff and ran the checks above.

[Copilot]

Pull request overview

This PR adjusts the typing contract of RemoteProgress._parse_progress_line() to support subclass overrides that return a value (without violating Liskov substitutability), while keeping the base implementation conceptually “void” and ensuring internal callers ignore the return value.

Changes:

• Broaden RemoteProgress._parse_progress_line() return annotation from None to Optional[object], and make the base implementation return None explicitly on all paths.
• Update RemoteProgress.new_message_handler() to ignore _parse_progress_line()’s return value (avoids returning a non-None type from a -> None handler).
• Add a regression test demonstrating a subclass override that returns a non-None value.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
git/util.py	Broadens _parse_progress_line() return typing and ensures call sites (message handler) don’t propagate that return value.
test/deprecation/test_remote_progress.py	Adds a regression test proving overrides may return non-None under the new base typing.

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[Byron]

Thanks, LGTM!