Handle mypy CI typing failure

Agent-Logs-Url: https://github.com/gitpython-developers/GitPython/sessions/3d3e7ffc-d3af-478e-9c6c-128731cdd102

Co-authored-by: Byron <63622+Byron@users.noreply.github.com>

Author: Copilot

git/objects/commit.py

@@ -463,14 +463,15 @@ def trailers_list(self) -> List[Tuple[str, str]]:
         return trailer_list
 
     @classmethod
-    def _interpret_trailers(cls, repo: "Repo", message: str, trailer_args: Sequence[str]) -> str:
+    def _interpret_trailers(cls, repo: "Repo", message: Union[str, bytes], trailer_args: Sequence[str]) -> str:
         cmd = [repo.git.GIT_PYTHON_GIT_EXECUTABLE, "interpret-trailers", *trailer_args]
         proc: Git.AutoInterrupt = repo.git.execute(  # type: ignore[call-overload]
             cmd,
             as_process=True,
             istream=PIPE,
         )
-        stdout_bytes, _ = proc.communicate(message.encode(cls.default_encoding, errors="strict"))
+        message_bytes = message if isinstance(message, bytes) else message.encode(cls.default_encoding, errors="strict")
+        stdout_bytes, _ = proc.communicate(message_bytes)
         finalize_process(proc)
         return stdout_bytes.decode(cls.default_encoding, errors="strict")