test: preserve hidden gitfiles when preparing submodule aliases

The Windows Python 3.8 partial CI log showed two failures in
`test_submodule_allows_existing_metadata_symlinks`: preparing update and
move aliases raised `PermissionError` before invoking GitPython. Git marks
the submodule's `.git` file hidden; Python's `write_text()` attempts to
recreate it, which Windows rejects for an existing hidden file.

Open that fixture file with `r+`, write the replacement target, and truncate
it. This preserves the hidden attribute while replacing the full contents.
The separate fixture that creates a previously absent gitfile is unchanged.

Validation: all six native/windows37 update, move, and remove alias modes
pass locally. Ruff lint, formatting, and `git diff --check` pass. The
Windows-specific file-attribute behavior will be verified by CI.

Author: codex

test/test_submodule.py

@@ -715,7 +715,11 @@ def test_submodule_allows_existing_metadata_symlinks(
     target = tmp_path / "outside"
     if kind == "alias":
         target = modules / "nested"
-        (root / "module/.git").write_text("gitdir: ../.git/modules/alias/module")
+        # Git hides .git on Windows; opening that existing file with O_CREAT
+        # fails even when it is writable. Preserve its attributes with r+.
+        with (root / "module/.git").open("r+", encoding="utf-8") as gitfile:
+            gitfile.write("gitdir: ../.git/modules/alias/module")
+            gitfile.truncate()
     else:
         link.rename(target)
     link.symlink_to(target, target_is_directory=target.is_dir())