config: resolve relative includes from relative config paths

[officialasishkumar]

Fixes #2103

Summary

• Normalize path-based config inputs before using them for include cycle detection.
• Resolve relative include.path entries from that normalized source path when the root config file was supplied as a relative path.
• Add regression coverage with a relative root config and a relative include cycle.

Testing

• .venv/bin/pytest --no-cov -q test/test_config.py
• .venv/bin/pytest --no-cov -q test/test_config.py test/test_repo.py::TestRepo::test_config_reader
• .venv/bin/pytest --no-cov -q test/test_refs.py::TestRefs::test_set_tracking_branch_with_import
• .venv/bin/ruff check git/config.py test/test_config.py
• .venv/bin/ruff format --check git/config.py test/test_config.py
• .venv/bin/mypy git/config.py
• git diff --check

[Copilot]

Pull request overview

Fixes relative [include] path = ... handling when the root config file is provided as a relative path (regression for #2103), by normalizing config file paths for both include-cycle detection and relative-include resolution.

Changes:

• Normalize initial config file inputs to absolute, normalized paths for include cycle detection.
• Resolve relative include.path values using the normalized absolute source config path (instead of asserting the source path is already absolute).
• Add a regression test covering a relative root config path with a relative include cycle.

Reviewed changes

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

File	Description
git/config.py	Normalizes config paths for seen and uses an absolute source path to resolve relative include paths without asserting the caller provided an absolute root path.
test/test_config.py	Adds a regression test ensuring relative root config paths correctly resolve relative includes and avoid include-cycle failures.

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