Some incremental git repository loadings are failing

Since the recent fix to the git loader for fetching incremental pack files, some repositories already loaded of forked from repositories already loaded fail with the following error when attempting to load them (again).

Sentry Issue: SWH-LOADER-GIT-15P

KeyError: [b'36ce1309339c36e1b23a4c930a04d4e97b93ea8e', b'aba6f30300265f7a5ff1cc3ce316cb99a3397fd5', b'6a61bad049c35245aa49256d91e0662d6ca54f9d']
(2 additional frame(s) were not displayed)
...
  File "swh/loader/git/loader.py", line 395, in get_contents
    for raw_obj in self.iter_objects(b"blob"):
  File "swh/loader/git/loader.py", line 385, in iter_objects
    PackData.from_file(self.pack_buffer, self.pack_size)
  File "dulwich/pack.py", line 1388, in _walk_all_chains
    for result in self._walk_ref_chains():
  File "dulwich/pack.py", line 1398, in _walk_ref_chains
    self._ensure_no_pending()
  File "dulwich/pack.py", line 1394, in _ensure_no_pending
    raise KeyError([sha_to_hex(s) for s in self._pending_ref])

Loading failure, updating to `failed` status