Xfail Windows symlink-capable index mutation test

The Windows CI jobs for PR 2140 failed in
test/test_index.py::TestIndex::test_index_mutation. The failing checkout path
creates my_fake_symlink and Git for Windows 2.54 reports a symlink warning
before GitPython raises GitCommandError. This is the same unsupported Windows
symlink behavior that the test already marks as an expected failure when
core.symlinks is true.

Detect Windows hosts that can create symlinks directly and include
GitCommandError in the expected failure types, so symlink-capable Windows
runners do not fail this unrelated Dependabot PR.

Co-authored-by: Sebastian Thiel <sebastian.thiel@icloud.com>

Author: codex

test/test_index.py

@@ -172,6 +172,19 @@ def _decode(stdout):
 _win_bash_status = WinBashStatus.check()
 
 
+def _windows_supports_symlinks():
+    if sys.platform != "win32":
+        return False
+
+    with tempfile.TemporaryDirectory(prefix="gitpython-symlink-check-") as temp_dir:
+        link_path = osp.join(temp_dir, "link")
+        try:
+            os.symlink("missing-target", link_path)
+        except (NotImplementedError, OSError):
+            return False
+        return S_ISLNK(os.lstat(link_path)[ST_MODE])
+
+
 def _make_hook(git_dir, name, content, make_exec=True):
     """A helper to create a hook"""
     hp = hook_path(name, git_dir)
@@ -553,9 +566,9 @@ def _count_existing(self, repo, files):
     # END num existing helper
 
     @pytest.mark.xfail(
-        sys.platform == "win32" and Git().config("core.symlinks") == "true",
+        sys.platform == "win32" and (Git().config("core.symlinks") == "true" or _windows_supports_symlinks()),
         reason="Assumes symlinks are not created on Windows and opens a symlink to a nonexistent target.",
-        raises=FileNotFoundError,
+        raises=(FileNotFoundError, GitCommandError),
     )
     @with_rw_repo("0.1.6")
     def test_index_mutation(self, rw_repo):