Make sure that multi-options are checked after splitting them with shlex

[Byron]

Fix for GHSA-x2qx-6953-8485

[Copilot]

Pull request overview

This PR addresses the GitPython security advisory GHSA-x2qx-6953-8485 by ensuring “multi-options” are validated for unsafe flags after they are split via shlex, preventing unsafe options from being hidden inside a single multi-option string.

Changes:

• Update clone option validation to run check_unsafe_options against the shlex-split multi-options list.
• Add regression tests for Repo.clone, Repo.clone_from, and Submodule.update to confirm unsafe options are caught when embedded in a combined multi-option payload.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
git/repo/base.py	Validates unsafe clone options against the split multi args instead of the raw multi_options strings.
test/test_clone.py	Adds tests ensuring unsafe options are rejected after splitting multi_options payload strings.
test/test_submodule.py	Adds a test ensuring submodule update clone options are checked after splitting combined payloads.

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