fix(repo): preserve literal Cygwin gitfile paths

The `test-cygwin` workflow's `test (fast)` job reported two
`InvalidGitRepositoryError` failures in the new submodule regression.
Cloning preserved the requested metadata directory, but reopening it
expanded the relative path read from the submodule's `.git` file.

Disable expansion when `find_submodule_git_dir()` converts gitfile paths
with `cygpath()`. These paths come from repository metadata and must be
used literally. This completes the Cygwin behavior for
`GHSA-fx3j-rwgx-fr94`.

Extend the existing regression to exercise Cygwin gitfile conversion on
every platform. The same two cases now reproduce locally before the fix
and pass afterward. Git's `read_gitfile_gently()` in `setup.c` at
`d38352cd43ab9745686d697872408bc3249a153f` likewise resolves the literal
gitfile path without environment expansion.

Validation:
- All 22 clone/submodule regression cases pass.
- `test/test_fun.py` and `test/test_submodule.py`: 177 passed, 3 skipped,
  and 1 expected failure. The one sandbox-blocked temporary-index test
  passed on rerun with access to this worktree's Git metadata directory.
- `ruff check` and `ruff format --check` pass for the changed files.
- `mypy` passes for all 46 checked source files.

Author: codex

git/repo/fun.py

@@ -151,7 +151,7 @@ def find_submodule_git_dir(d: PathLike) -> Optional[PathLike]:
         # Cygwin creates submodules prefixed with `/cygdrive/...`.
         # Cygwin git understands Cygwin paths much better than Windows ones.
         # Also the Cygwin tests are assuming Cygwin paths.
-        path = cygpath(path)
+        path = cygpath(path, expand_vars=False)
     if not osp.isabs(path):
         path = osp.normpath(osp.join(osp.dirname(d), path))
     return path if is_git_dir(path) else None

test/test_submodule.py

@@ -70,6 +70,11 @@ def test_submodule_update_preserves_literal_name(tmp_path, monkeypatch, caplog,
 
             modules_dir = Path(clone.git_dir, "modules")
             assert {path.name for path in modules_dir.iterdir()} == {name}
+            # Exercise relative gitfile conversion on every platform.
+            with mock.patch.object(Git, "is_cygwin", return_value=True):
+                resolved_git_dir = find_submodule_git_dir(Path(clone.working_tree_dir, "module", ".git"))
+            assert resolved_git_dir is not None
+            assert (modules_dir / name).samefile(resolved_git_dir)
             with clone.submodules[0].module() as module:
                 assert (modules_dir / name).samefile(module.git_dir)
                 assert module.head.commit == source.head.commit