test: copy prepared submodule baselines for each case

`movable_submodule` repeatedly initialized and committed the same source
and parent before each rejection or mutation test. Prepare each logical
name once per module and copy the parent with `shutil.copytree` instead.
Relative gitfiles and `core.worktree` settings remain usable after copying;
only the immutable source URL is shared. Every case gets fresh wrappers
and independent refs, index, config, objects, and worktree files.

Add an isolation check for distinct writable files and baseline preservation
after edits and branch creation. Existing security snapshots remain intact.
All 333 top-level submodule cases passed with GixPython first in 88.63
seconds wall time, then CLI in 178.75 seconds. Ruff checks passed.

Author: codex

test/test_submodule.py

@@ -3,6 +3,7 @@
 
 import contextlib
 import gc
+from functools import lru_cache
 import os
 import os.path as osp
 from pathlib import Path
@@ -145,22 +146,49 @@ def test_submodule_cli_lifecycle(tmp_path, monkeypatch, object_format, ref_forma
         assert not parent.submodules
 
 
+@pytest.fixture(scope="module")
+def movable_submodule_baseline(tmp_path_factory):
+    """Prepare each logical-name layout once; only copies are handed to tests."""
+
+    @lru_cache(maxsize=None)
+    def prepare(name):
+        root = tmp_path_factory.mktemp("movable-submodule")
+        with git.Repo.init(root / "source") as source, git.Repo.init(root / "parent") as parent:
+            source.git.symbolic_ref("HEAD", "refs/heads/master")
+            (root / "source/file").write_text("content", encoding="utf-8")
+            source.index.add(["file"])
+            source.index.commit("Create source")
+            with _patch_git_config("protocol.file.allow", "always"):
+                submodule = parent.create_submodule(name, "module", source.working_tree_dir)
+            parent.index.commit("Create submodule")
+            submodule.module().close()
+        return root / "parent"
+
+    return prepare
+
+
 @pytest.fixture
-def movable_submodule(tmp_path, request):
-    """Create a committed local submodule whose logical name stays fixed when moved."""
-    with git.Repo.init(tmp_path / "source") as source, git.Repo.init(tmp_path / "parent") as parent:
-        source.git.symbolic_ref("HEAD", "refs/heads/master")
-        (tmp_path / "source" / "file").write_text("content", encoding="utf-8")
-        source.index.add(["file"])
-        source.index.commit("Create source")
-        with _patch_git_config("protocol.file.allow", "always"):
-            submodule = parent.create_submodule(
-                getattr(request, "param", "logical-name"), "module", source.working_tree_dir
-            )
-        parent.index.commit("Create submodule")
-        # Release clone handles before Windows moves the checkout.
-        submodule.module().close()
-        yield submodule
+def movable_submodule(tmp_path, request, movable_submodule_baseline):
+    """Give each test independent writable metadata and a fresh Python wrapper."""
+    baseline = movable_submodule_baseline(getattr(request, "param", "logical-name"))
+    shutil.copytree(baseline, tmp_path / "parent", symlinks=True)
+    with git.Repo(tmp_path / "parent") as parent:
+        yield parent.submodules[0]
+
+
+def test_movable_submodule_copies_are_isolated(movable_submodule, movable_submodule_baseline):
+    sm = movable_submodule
+    baseline = movable_submodule_baseline(sm.name)
+    for relative in (".git/index", ".git/config", ".git/modules/logical-name/config", "module/file"):
+        copy = Path(sm.repo.working_tree_dir, relative)
+        original = baseline / relative
+        assert copy.read_bytes() == original.read_bytes()
+        assert not copy.samefile(original)
+    Path(sm.abspath, "file").write_text("changed", encoding="utf-8")
+    sm.repo.create_head("copy-only")
+    assert (baseline / "module/file").read_text(encoding="utf-8") == "content"
+    with git.Repo(baseline) as original:
+        assert "copy-only" not in original.heads
 
 
 def _move_snapshot(submodule):