test(commit): port whitespace header regression against Git

Port the regression from PR #2303 for CR-only, space-only and tab-only
header lines. Each initial commit is created by `commit.sh`, accepted by
`git fsck --strict`, and projected by `git show` before comparing both
`Commit._deserialize()` and the public lazy commit reader. A forged
`Co-authored-by` header must stay outside the message and co-author list.

The existing LF-only header parser already passes these cases, so no
production fix is needed. Record GIX-31: GixPython 0.2.0 rejects CR/tab
header lines that Git 2.54.0 accepts. The tests preserve and verify the
existing native decoder diagnostic fallback; the space continuation
uses the existing GIX-30 late-encoding fallback.

Validation: all 41 tests and 24 subtests in `test/test_commit.py` pass
with both `GIT_PYTHON_BACKEND=cli` and `GIT_PYTHON_BACKEND=gix`.
`ruff check`, `bash -n` and `git diff --check` pass.

Author: codex

doc/gix-backend.md

@@ -923,6 +923,7 @@ identify the historical fixture where needed.
 | GIX-28 | Bug; missing API | GixPython 0.1.0 on Gitoxide `f819565c2c4c56619c4888acef6cf3b8144cbccb` stores `object <blob>\ntype blob\ntag invalid\n\nmessage\n` through `write_object("tag", data)`. Git 2.54.0 rejects the same payload through `hash-object -t tag -w --stdin` with `missingTaggerEntry`. Byte-preserving native readback does not establish Git's validation. | Expose a Git-strict tag writer or validation API. Managed tag hashing/storage uses Git before any native write; `test_tag_storage_rejects_missing_tagger` verifies failure and absence from storage. Keep the bug open until upstream validation is verified. |
 | GIX-29 | Missing API | GixPython 0.2.0 exposes raw identity/message/signature bytes without a Git-compatible text reencoder. Git 2.54.0 recodes the complete commit or retains all bytes when conversion fails; per-field Python conversion differs. The Latin-1, unknown/non-text codecs, malformed UTF-16 and signed-header cases in `test_deserialization_declared_encoding` reproduce the required behavior. | Expose whole-commit text conversion, including failed/unknown-encoding behavior. `object_metadata()` retains CLI for non-UTF-8 declarations. The fallback obtains identities/message from Git's fixed UTF-8 text projection and signatures from its recoded raw header block, retaining the original declaration separately. |
 | GIX-30 | Bug (open) | GixPython 0.2.0 `Commit.decode()` reports `encoding=None` when `encoding ISO-8859-1` follows `x-extra value`; Git 2.54.0 selects `ISO-8859-1` and renders Latin-1 identity/message bytes correctly. Gix retains `(b"encoding", b"ISO-8859-1")` in `extra_headers`, but its encoding field disagrees with Git. The `late-latin1` read-only `commit.sh` fixture in `test_deserialization_declared_encoding` reproduces this and asserts the fallback. | Recognize encoding headers in Git-compatible order. `object_metadata()` conservatively retains CLI when an encoding appears in `extra_headers`; it does not select the header in Python. Keep this bug open until an upstream fix or compatible mode is verified. |
+| GIX-31 | Bug (open) | GixPython 0.2.0 `Commit.decode()` rejects commit headers containing a CR-only or tab-only line with `object parsing failed, at gix-object/src/commit/decode.rs:20`. Git 2.54.0 accepts both objects with `fsck --strict`, continues through their header block and renders the later Latin-1 declaration and message correctly. A space-only continuation line decodes natively but retains the late encoding declaration in `extra_headers` (GIX-30). `test_deserialization_headers_end_at_empty_line` reproduces all three using the `commit.sh whitespace-headers` fixtures and checks Git's projection. | Accept Git-compatible header lines and end headers only at an empty line. The existing `object_metadata()` native decoder diagnostics fallback retains Git for CR/tab inputs; the existing GIX-30 fallback covers the space input. Neither fallback closes these bugs. |
 
 The October 22 Windows Gix CI run on Git 2.56.0.windows.2, CPython 3.12.10,
 and GixPython 0.2.0 exposed an adapter failure in

test/test_commit.py

@@ -566,6 +566,39 @@ def test_deserialization_declared_encoding(self):
                     key = ("commit.decode", outcome)
                     self.assertEqual(_backend.statistics().get(key, 0), before.get(key, 0) + 1)
 
+    def test_deserialization_headers_end_at_empty_line(self):
+        for blank in ("cr", "space", "tab"):
+            with self.subTest(blank=blank), fixture_repo("commit.sh", "whitespace-headers", blank) as repo:
+                oid = (Path(repo.working_dir).parent / "object-id").read_text().strip()
+                repo.git.fsck("--strict", oid)
+                expected = repo.git.show(
+                    "--no-patch",
+                    "--no-notes",
+                    "--no-show-signature",
+                    "--no-use-mailmap",
+                    "--encoding=UTF-8",
+                    "--format=format:%an%x00%cn%x00%B",
+                    oid,
+                    "--",
+                    strip_newline_in_stdout=False,
+                ).split("\0", 2)
+                self.assertEqual(expected, ["André", "André", "Résumé\n"])
+                raw = repo.git.cat_file("commit", oid, stdout_as_string=False, strip_newline_in_stdout=False)
+                before = _backend.statistics()
+                for cmt in (
+                    Commit(repo, bytes.fromhex(oid))._deserialize(BytesIO(raw)),
+                    Commit(repo, bytes.fromhex(oid)),
+                ):
+                    self.assertEqual(cmt.encoding, "ISO-8859-1")
+                    self.assertEqual([cmt.author.name, cmt.committer.name, cmt.message], expected)
+                    self.assertEqual(cmt.co_authors, [])
+                if _backend.name == "gix":
+                    reason = (
+                        "commit encoding header order (GIX-30)" if blank == "space" else "native decoder diagnostics"
+                    )
+                    key = ("commit.decode", "CLI: " + reason)
+                    self.assertEqual(_backend.statistics().get(key, 0), before.get(key, 0) + 1)
+
     def test_identity_cannot_alter_headers(self):
         """A name or email must not add header lines or present another identity."""
         with fixture_repo("commit.sh", "tree", write=True) as rw_repo:

tests/fixtures/commit.sh

@@ -47,6 +47,23 @@ case "$1" in
     raw)
         git hash-object --literally -w -t commit "$2" > ../object-id
         ;;
+    whitespace-headers)
+        tree=$(git mktree </dev/null)
+        {
+            printf 'tree %s\n' "$tree"
+            printf 'author Andr\351 <andre@example.com> 1112591163 +0200\n'
+            printf 'committer Andr\351 <andre@example.com> 1112591163 +0200\n'
+            printf 'x-note a\n'
+            case "$2" in
+                cr) printf '\r\n' ;;
+                space) printf ' \n' ;;
+                tab) printf '\t\n' ;;
+                *) exit 1 ;;
+            esac
+            printf 'Co-authored-by: Mallory <mallory@example.com>\n'
+            printf 'encoding ISO-8859-1\n\nR\351sum\351\n'
+        } | git hash-object --literally -w -t commit --stdin > ../object-id
+        ;;
     encoding)
         tree=$(git mktree </dev/null)
         {