perf(gix): reuse object streams for serialization readback

Commit and tree serialization wrote native objects, then launched a fresh
`git cat-file` process to read their bytes back. Read through `Repo.odb.stream`
instead so Gix can serve the object and the CLI backend can reuse its batch
reader. Existing large-object and unsupported-storage fallbacks still apply.

The fixed probe serializing one commit and one tree drops from 2 CLI launches
to 0 with Gix, excluding fixture setup. Regression assertions compare the
serialized bytes with Git and prohibit CLI launches during native readback.
The earlier whole-suite inventory contained 779 matching readbacks; that is
an optimization target, not a newly measured suite-wide reduction.

Validation: the affected commit, tree, object/index, merge and serialization
selection passed with Gix first (28 tests, 8.04s), then CLI (26 tests, one
skipped, 19.05s). Ruff lint/format and `git diff --check` passed.

Author: codex

doc/gix-backend.md

@@ -128,6 +128,7 @@ The historical full-suite counts above predate this correction. A constant
 | --- | --- | --- |
 | `Git.get_object_header`, ODB `info` | Object resolution and native header lookup | Custom storage, unsupported revision grammar |
 | `Git.stream_object_data`, ODB `stream` | Object bytes in an independent stream | Objects larger than 8 MiB; GIX-2 |
+| Commit/tree serialization readback | Existing ODB stream; no separate `cat-file` process | The same storage and large-object fallbacks as ODB reads |
 | `Repo` opening/discovery | Native storage, worktree, object format and empty-tree metadata | Repository-format validation, unsupported layouts/environment and native validation gaps; GIX-1/14/19 |
 | `Repo.rev_parse`, `rev_parse` metadata queries | Revision lookup, common directory, object format, bare flag, worktree root, fixed `modules`/`COMMIT_EDITMSG` locations | Repository-format validation, other metadata paths, symlinked metadata leaves, message searches and describe forms; GIX-1/14/17/22 |
 | Tree enumeration, `ls_tree` | Native tree entries with modes, names, and object IDs | Other command options |

git/objects/commit.py

@@ -850,13 +850,8 @@ def _serialize(self, stream: BytesIO) -> "Commit":
             signature = self.gpgsig if any(self.binsha) else ""
         if signature:
             raise ValueError("Writing an existing commit signature is unsupported")
-        oid = self._calculate_sha_(self.repo, self).hex()
-        Git._check_operand(oid, "commit object ID")
-        stream.write(
-            self.repo.git._call_process_safe(
-                "cat_file", "commit", oid, stdout_as_string=False, strip_newline_in_stdout=False
-            )
-        )
+        oid = self._calculate_sha_(self.repo, self)
+        stream.write(self.repo.odb.stream(oid).read())
         return self
 
     def _deserialize(self, stream: BytesIO) -> "Commit":

git/objects/tree.py

@@ -420,11 +420,7 @@ def _serialize(self, stream: "BytesIO") -> "Tree":
             source.seek(0)
             oid = self.repo.git._call_process_safe("mktree", "-z", "--missing", istream=source)
         Git._check_operand(oid, "tree object ID")
-        stream.write(
-            self.repo.git._call_process_safe(
-                "cat_file", "tree", oid, stdout_as_string=False, strip_newline_in_stdout=False
-            )
-        )
+        stream.write(self.repo.odb.stream(to_bin_sha(oid)).read())
         return self
 
 

test/test_gix_backend.py

@@ -237,6 +237,17 @@ def test_native_tree_and_commit_bytes_match_cli(repo):
     native_tree = index.write_tree()
     commit = index.commit("message", author=actor, committer=actor, skip_hooks=True)
     native = commit.replace(message="a changed message")
+    expected = [
+        repo.git.cat_file(obj.type, obj.hexsha, stdout_as_string=False, strip_newline_in_stdout=False)
+        for obj in (commit, native_tree)
+    ]
+    count = _backend.statistics().get(("Git.execute", "CLI process"), 0)
+    with patch.object(Git, "execute", side_effect=AssertionError("unexpected serialization CLI call")):
+        for obj, content in zip((commit, native_tree), expected):
+            stream = BytesIO()
+            obj._serialize(stream)
+            assert stream.getvalue() == content
+    assert _backend.statistics().get(("Git.execute", "CLI process"), 0) == count
     with patch.object(_backend, "gix", None):
         assert index.write_tree().hexsha == native_tree.hexsha
         assert commit.replace(message="a changed message").hexsha == native.hexsha