Add test for cargo publish with gitignored nested git repositories

[Copilot]

Adds regression test for issue rust-lang#16547 where cargo publish was incorrectly flagging files in gitignored nested repositories as uncommitted changes.

Test scenario

• Main git repository with a Cargo package
• Nested git repository in tests/fixtures/ (gitignored)
• Files committed in the nested repository
• cargo publish should succeed without "uncommitted changes" errors

The test currently passes, verifying the issue is resolved. If the bug were present, publish would fail with:

ERROR: 1 files in the working directory contain changes that were not yet committed into git:
tests/fixtures/nested-repo/.github/workflow.yml

This mirrors the real-world case from gitoxide where test fixtures are themselves git repositories.

Original prompt

Look at this issue and create a test-case for cargo publish which involves a .gitignored repository within the crate to be published. It should reproduce the issue, calling ignored files uncommitted changes.

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