`_ensure_no_pending` failed

Sentry Issues: SWH-LOADER-GIT-1H3, SWH-LOADER-GIT-20A

KeyError: [b'6bebd96b67aae589821de152fe01cb34f3be618d', b'630617279be70604c474ac771c682f6bccbd2858', b'106230d2392d198319ec982bdbad714b8fa557d6']
(2 additional frame(s) were not displayed)
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  File "swh/loader/git/loader.py", line 485, in get_contents
    for raw_obj in self.iter_objects(b"blob"):
  File "swh/loader/git/loader.py", line 475, in iter_objects
    resolve_ext_ref=self._resolve_ext_ref,
  File "dulwich/pack.py", line 1388, in _walk_all_chains
    for result in self._walk_ref_chains():
  File "dulwich/pack.py", line 1417, 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])

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