Keep submodule checkouts away from the worktree root

Preserve the submodule-specific root-path rejection after resolving checkout paths for symlink containment.

Add a regression proving update rejects a checkout path at the parent repository root.

Validation: .venv/bin/python -m pytest test/test_submodule.py -q; pre-commit run --files git/objects/submodule/base.py test/test_submodule.py; mypy git/objects/submodule/base.py; git diff --check.

Author: codex

git/objects/submodule/base.py

@@ -419,7 +419,8 @@ def abspath(self) -> PathLike:
         path = super().abspath
         root = self.repo.working_tree_dir
         assert root is not None
-        _to_relative_path(osp.realpath(root), osp.realpath(path))
+        if _to_relative_path(osp.realpath(root), osp.realpath(path)) == ".":
+            raise ValueError("Submodule checkout path must not be the repository root")
         return path
 
     @classmethod

test/test_submodule.py

@@ -1400,6 +1400,14 @@ def test_update_rejects_checkout_path_through_outside_symlink(self, rwdir):
             with pytest.raises(ValueError, match="is not in repository"):
                 submodule.update(init=True)
 
+    @with_rw_directory
+    def test_update_rejects_checkout_path_at_parent_root(self, rwdir):
+        parent = git.Repo.init(osp.join(rwdir, "parent"))
+        submodule = Submodule(parent, Submodule.NULL_BIN_SHA, name="module", path=".", url="unused")
+
+        with pytest.raises(ValueError, match="must not be the repository root"):
+            submodule.update(init=True)
+
     @skipUnless(sys.platform == "win32", "Specifically for Windows.")
     @with_rw_directory
     def test_to_relative_path_windows_path_kinds(self, rwdir):