fix(gix): defer compatibility object formats to Git

GixPython opens repositories with `extensions.compatObjectFormat`, but native
translation and object-write compatibility have not been validated. The local
Git build reports that compatibility hash support requires Rust, so its failed
translation probe does not establish a defect in GixPython's object mappings.

Reject this capability in the shared native repository adapter before reads or
mutations. Git retains responsibility for repositories using the extension,
and the coverage report identifies the conservative guard as `GIX-19`.

The new SHA-1 and SHA-256 regression cases failed before this guard. They verify
CLI dispatch for reads and writes without depending on Git's optional
compatibility support, and forbid any native write attempt. All 36 focused
backend tests and Ruff lint/format checks pass.

Author: codex

git/_backend.py

@@ -83,8 +83,11 @@ def _repository(command: Any, env: Dict[str, Any], *, query_config: bool = False
             ["core.fsmonitor=false", "gc.auto=0", "maintenance.auto=false", "core.hooksPath=" + os.devnull]
         )
     repo = gix.open_opts(path, options)
-    if repo.config_snapshot().string("extensions.refStorage") == b"reftable":
+    snapshot = repo.config_snapshot()
+    if snapshot.string("extensions.refStorage") == b"reftable":
         raise _Unsupported("reftable (GIX-1)")
+    if snapshot.string("extensions.compatObjectFormat") is not None:
+        raise _Unsupported("compatibility object format (GIX-19)")
     if effective.get("GIT_WORK_TREE"):
         workdir = effective["GIT_WORK_TREE"]
         if not os.path.isabs(workdir):

test/test_gix_backend.py

@@ -110,6 +110,20 @@ def test_unknown_command_and_storage_environment_use_cli(repo):
         assert cli.call_count == 3
 
 
+def test_compatibility_object_format_uses_cli_before_writing(repo):
+    repo.git.config("core.repositoryFormatVersion", "1")
+    repo.git.config("extensions.compatObjectFormat", "sha256" if repo.object_format == "sha1" else "sha1")
+    # Check dispatch without requiring Git's optional compatibility-hash support.
+    with patch.object(_backend, "_write", side_effect=AssertionError("unexpected native mutation")):
+        with patch.object(Git, "execute", return_value="fallback") as cli:
+            assert repo.git._call_process_safe("rev_parse", "--show-object-format") == "fallback"
+            assert (
+                repo.git._call_process_safe("hash_object", "-t", "blob", "-w", "--stdin", istream=BytesIO(b"content"))
+                == "fallback"
+            )
+            assert cli.call_count == 2
+
+
 def test_native_command_queries_match_cli(repo, monkeypatch):
     Path(repo.working_dir, "file").write_bytes(b"content")
     repo.index.add(["file"])