test: cover Cygwin index filename restrictions

The partial Cygwin fast-suite log exposed a failure in
`test_valid_unusual_index_names_round_trip`: native Git omitted a literal
backslash filename. Cygwin Git recognizes Windows separators and applies
NTFS path protection, even though Python reports a POSIX platform.

Move that case into the existing unsupported-name assertions for Cygwin.
Check that GitPython raises `ValueError`, preserves the published index,
and removes its lock. Other POSIX systems retain the round-trip case;
Windows retains the control-character and colon rejection cases. Document
the Cygwin restriction in `changes.rst`.

Validation: three focused index tests pass locally. The Cygwin and Windows
test branches pass with only Git's ignored-record behavior simulated;
native Cygwin verification awaits CI. Ruff lint, formatting, and
`git diff --check` pass.

Author: codex

doc/source/changes.rst

@@ -65,8 +65,9 @@ API changes
   indexes isolate tree/merge operations from the real index and working tree.
   Git's platform-specific index filename restrictions apply. Unsupported entries
   raise ``ValueError`` before the original index changes, including names with
-  colons or control characters on Windows. Tree objects can still contain names
-  that the working tree or index cannot represent.
+  colons or control characters on Windows and backslashes with Cygwin Git.
+  Tree objects can still contain names that the working tree or index cannot
+  represent.
   ``version`` is read-only. ``from_tree()`` accepts ``trivial``, ``aggressive``,
   and ``verbose`` options; arbitrary ``read-tree`` keyword forwarding is removed.
 * Standalone binary tree parsers, serializers, and multi-tree traversal helpers

test/test_index.py

@@ -271,23 +271,28 @@ def test_index_add_and_write_tree_reject_gitmodules_fallback_symlinks(self, rw_d
 
     def test_valid_unusual_index_names_round_trip(self):
         names = ["a b", "--option", "dir/.gitignore", "café"]
-        windows_unsupported = ["a\nb", "a\tb", "name:value"]
+        unsupported = ["a\nb", "a\tb", "name:value"]
         if os.name != "nt":
-            names.extend([*windows_unsupported, "a\\b", "\udc9f"])
+            names.extend([*unsupported, "\udc9f"])
+            unsupported = []
+            if sys.platform == "cygwin":
+                # Cygwin Git applies NTFS protection to backslash separators.
+                unsupported.append("a\\b")
+            else:
+                names.append("a\\b")
         with tempfile.TemporaryDirectory() as directory:
             index = IndexFile(self.rorepo, Path(directory, "index"))
             index.entries = {(name, 0): IndexEntry((0o100644, b"a" * 20, 0, name)) for name in names}
             index.write()
             assert sorted(entry.path for entry in index.update().entries.values()) == sorted(names)
-            if os.name == "nt":
-                before = Path(index.path).read_bytes()
-                for name in windows_unsupported:
-                    index.entries[(name, 0)] = IndexEntry((0o100644, b"a" * 20, 0, name))
-                    with pytest.raises(ValueError, match="Git did not retain"):
-                        index.write()
-                    assert Path(index.path).read_bytes() == before
-                    assert not Path(str(index.path) + ".lock").exists()
-                    del index.entries[(name, 0)]
+            before = Path(index.path).read_bytes()
+            for name in unsupported:
+                index.entries[(name, 0)] = IndexEntry((0o100644, b"a" * 20, 0, name))
+                with pytest.raises(ValueError, match="Git did not retain"):
+                    index.write()
+                assert Path(index.path).read_bytes() == before
+                assert not Path(str(index.path) + ".lock").exists()
+                del index.entries[(name, 0)]
 
     @ddt.data("write", "write_tree")
     def test_index_rejects_silently_ignored_entries_atomically(self, operation):