feat(gix): query the index version natively

Use the native index version for `IndexFile.version`'s managed query. Reuse the index capability guards; actual `update-index` mutations continue to use Git.

The staged version passes Ruff and Python syntax checks. Its SHA-1/SHA-256 native smoke and focused pytest selection pass: 16 passed in 4.01s.

Author: codex

git/_backend.py

@@ -344,13 +344,20 @@ def _ls_files(repo: Any, args: List[str], kwargs: Dict[str, Any]) -> bytes:
     return b"".join(output)
 
 
+def _update_index(repo: Any, args: List[str], kwargs: Dict[str, Any]) -> bytes:
+    if args != ["--show-index-version"]:
+        raise _Unsupported("index mutation options")
+    return str(_index(repo, kwargs).version()).encode("ascii") + b"\n"
+
+
 _HANDLERS: Dict[str, Callable[[Any, List[str], Dict[str, Any]], bytes]] = {
     "rev_parse": _rev_parse,
     "ls_tree": _ls_tree,
     "symbolic_ref": _symbolic_ref,
     "for_each_ref": _for_each_ref,
     "merge_base": _merge_base,
     "ls_files": _ls_files,
+    "update_index": _update_index,
     "config": _config,
     "worktree": _worktree,
 }