fix(commit): parse the encoding header when deserializing

`Commit._deserialize` compared a 10-byte slice of each stripped header
line against the 9-byte literal `b"encoding "`, so the comparison never
matched and the branch reading a commit's declared encoding was
unreachable. Every commit parsed from the object database was decoded
with the default UTF-8: `Commit.encoding` reported `UTF-8` regardless of
the header, and the author, committer, and message of a commit written
under `i18n.commitencoding` came back with U+FFFD replacement characters
while `git log` shows them correctly. Re-serializing such a commit also
dropped its `encoding` header and stored the garbled text.

Compare the right slice length so the header is read. Adopt the declared
encoding only when Python has a text codec for it: the identity lines
are decoded with `errors="replace"`, which does not guard against an
unknown codec name, so a commit declaring one would otherwise raise
`LookupError` on attribute access. An unknown name keeps the previous
behavior (the default encoding) and logs the error.

Validated against a repository committed under `i18n.commitencoding =
ISO-8859-1`: `commit.encoding`, `commit.author.name`, and
`commit.message` now match what `git log` shows. Adds regression tests
in `test/test_commit.py` that fail before the change.

Author: Keerthana-64

git/objects/commit.py

@@ -896,8 +896,17 @@ def _deserialize(self, stream: BytesIO) -> "Commit":
         enc = next_line
         buf = enc.strip()
         while buf:
-            if buf[0:10] == b"encoding ":
-                self.encoding = buf[buf.find(b" ") + 1 :].decode(self.encoding, "ignore")
+            if buf[0:9] == b"encoding ":
+                encoding = buf[buf.find(b" ") + 1 :].decode(self.encoding, "ignore")
+                try:
+                    # Probe one byte: decoding b"" skips the codec lookup entirely.
+                    b" ".decode(encoding, "replace")
+                except (LookupError, ValueError):
+                    # The encoding is not a text encoding Python knows; keep the
+                    # default so the identity and message lines still decode below.
+                    _logger.error("Ignoring unknown commit encoding %r", encoding)
+                else:
+                    self.encoding = encoding
             elif buf[0:7] == b"gpgsig ":
                 sig_lines = [buf[buf.find(b" ") + 1 :] + b"\n"]
                 is_next_header = False

test/test_commit.py

@@ -458,6 +458,41 @@ def test_invalid_commit(self):
         self.assertEqual(cmt.author.name, "E.Azer Ko�o�o�oculu", cmt.author.name)
         self.assertEqual(cmt.author.email, "azer@kodfabrik.com", cmt.author.email)
 
+    def test_deserialization_declared_encoding(self):
+        # The "encoding" header names the encoding of the identity and message bytes.
+        data = (
+            b"tree 3385f7b1e5f91930a3bd3e07105e113fca29981d\n"
+            b"author Andr\xe9 <andre@example.com> 1112591163 +0200\n"
+            b"committer Andr\xe9 <andre@example.com> 1112591163 +0200\n"
+            b"encoding ISO-8859-1\n"
+            b"\n"
+            b"R\xe9sum\xe9\n"
+        )
+        cmt = self.rorepo.commit()
+        cmt._deserialize(BytesIO(data))
+
+        self.assertEqual(cmt.encoding, "ISO-8859-1")
+        self.assertEqual(cmt.author.name, "André")
+        self.assertEqual(cmt.committer.name, "André")
+        self.assertEqual(cmt.message, "Résumé\n")
+
+    def test_deserialization_unknown_declared_encoding(self):
+        # An encoding Python has no text codec for falls back to the default encoding.
+        data = (
+            b"tree 3385f7b1e5f91930a3bd3e07105e113fca29981d\n"
+            b"author Andr\xe9 <andre@example.com> 1112591163 +0200\n"
+            b"committer Andr\xe9 <andre@example.com> 1112591163 +0200\n"
+            b"encoding pony-speak\n"
+            b"\n"
+            b"R\xe9sum\xe9\n"
+        )
+        cmt = self.rorepo.commit()
+        cmt._deserialize(BytesIO(data))
+
+        self.assertEqual(cmt.encoding, cmt.default_encoding)
+        self.assertEqual(cmt.author.name, "Andr�")
+        self.assertEqual(cmt.message, "R�sum�\n")
+
     @with_rw_directory
     def test_identity_cannot_alter_headers(self, rw_dir):
         """A name or email must not add header lines or present another identity."""