fix: allow relative config paths with includes (#2103)

GitConfigParser asserted while resolving a relative include whenever the
top-level configuration file had itself been supplied as a relative path.
A regression test constructs that combination and verifies the included
value is available.

Normalize path-based inputs to absolute paths before reading them. This
lets relative includes resolve against the containing file and ensures
cycle detection uses the same canonical spelling for top-level and
included paths.

This matches Git's documented include behavior and the relative and
chained-relative coverage in t/t1305-config-include.sh.

Validation:
- uv run pytest -q test/test_config.py
- uv run pre-commit run --files git/config.py test/test_config.py
- uv run mypy git/config.py
- git diff --check
- full pytest: 631 passed, 75 skipped, 34 unrelated environment failures
  (uninitialized nested submodules and missing legacy master branch)

Author: codex

git/config.py

@@ -634,6 +634,8 @@ def read(self) -> None:  # type: ignore[override]
             files_to_read = list(self._file_or_files)
         # END ensure we have a copy of the paths to handle
 
+        files_to_read = [osp.abspath(path) if isinstance(path, (str, os.PathLike)) else path for path in files_to_read]
+
         seen = set(files_to_read)
         num_read_include_files = 0
         while files_to_read:

test/test_config.py

@@ -310,6 +310,20 @@ def check_test_value(cr, value):
         with GitConfigParser(fpa, read_only=True) as cr:
             check_test_value(cr, tv)
 
+    @with_rw_directory
+    def test_config_relative_path_include(self, rw_dir):
+        included_path = osp.join(rw_dir, "included")
+        with GitConfigParser(included_path, read_only=False) as cw:
+            cw.set_value("included", "value", "included")
+
+        config_path = osp.join(rw_dir, "config")
+        with GitConfigParser(config_path, read_only=False) as cw:
+            cw.set_value("include", "path", "included")
+
+        relative_config_path = osp.relpath(config_path)
+        with GitConfigParser(relative_config_path, read_only=True) as cr:
+            assert cr.get_value("included", "value") == "included"
+
     @with_rw_directory
     def test_multiple_include_paths_with_same_key(self, rw_dir):
         """Test that multiple 'path' entries under [include] are all respected.