Handle relative include.path when root config is provided as a relative path

[Copilot]

Git config allows relative include paths to be resolved relative to the config file containing the directive. GitPython currently asserts the source config path is absolute during include resolution, which raises AssertionError for valid setups when the parser is initialized with a relative config path.

• What changed

  • Updated include resolution in GitConfigParser.read() to normalize the current config file path to absolute when needed before resolving relative includes.
  • Removed the absolute-path assertion that caused valid relative-path configurations to fail.

• Regression coverage

  • Added a focused test in test/test_config.py to verify:
    • main config includes path = b
    • main config is opened via a relative path
    • included values are successfully loaded from b

• Behavioral impact

  • Relative include semantics now align with git’s documented behavior even when the initial config path passed to GitConfigParser is relative.
  • Existing include cycle/seen-path handling remains unchanged.

# before: could raise AssertionError if main config path is relative
with GitConfigParser(os.path.relpath(main_cfg), read_only=True) as cr:
    cr.get_value("section", "key")

# now: relative include in main_cfg resolves correctly

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