Match git check-ignore for tracked ignored paths

[Sebastian Thiel (Byron)]

Codex created this PR on behalf of Byron. Byron will review it.

Fixes #2562

Summary

• suppress gix exclude query ignore matches for paths that are present in the index, matching git check-ignore
• preserve pathspec expansion behavior for tracked entries, exclusions, top magic, and attribute-qualified pathspecs
• add the tracked-under-ignored-directory case to the gix-worktree ignore baseline fixture, with baseline input/output files written in the fixture execution directory

Git baseline

For a repository with .gitignore containing bin/ and force-added src/bin/stub_gen.rs, git check-ignore -vn --stdin reports no ignore pattern for:

• src/bin
• src/bin/
• src/bin/stub_gen.rs

It still reports .gitignore:1:bin/ for untracked ignored paths:

• src/bin/extra.txt
• other/bin
• other/bin/file.txt

gix status --ignored matching --no-write already matched Git for this repro, so the fix is scoped to gix exclude query rather than dirwalk.

Validation

• cargo test -p gitoxide-core --lib --quiet
• GIX_TEST_IGNORE_ARCHIVES=1 cargo test -p gix-worktree-tests worktree::stack::ignore --quiet

[Copilot]

Pull request overview

This PR updates ignore/exclude querying to match git check-ignore semantics for paths that are ignored but tracked (or contain tracked entries), and extends the fixture baseline to cover the “tracked under ignored directory” scenario described in #2562.

Changes:

• Update the gitoxide-core exclude query implementation to suppress ignore matches for tracked paths (and tracked directories) based on index presence, aligning output with git check-ignore.
• Extend the gix-worktree ignore baseline fixture to include ignored directories containing tracked files, and adjust the test harness to read baseline inputs/outputs from the fixture execution directory.
• Add targeted unit tests in gitoxide-core to lock in expected behavior (tracked paths suppression, stdin path relativity, and pathspec expansion preservation).

Reviewed changes

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

File	Description
gix-worktree/tests/worktree/stack/ignore.rs	Adjusts baseline test logic to suppress ignore matches when the index indicates the path is tracked (or a tracked directory).
gix-worktree/tests/fixtures/make_ignore_and_attributes_setup.sh	Updates fixture repo setup and baseline generation to include the tracked-under-ignored-directory case and to emit baseline files in the fixture execution dir.
gitoxide-core/src/repository/exclude.rs	Implements index-based suppression of ignore matches and adds tests to verify parity with git check-ignore and preserve pathspec-related behavior.