scrubber crashes with "Branch aliases unresolved: b'HEAD' -> b'refs/heads/master'"

https://sentry.softwareheritage.org/organizations/swh/issues/10591/?referrer=phabricator_plugin

ValueError: ("Branch aliases unresolved: b'HEAD' -> b'refs/heads/master'", [(b'HEAD', b'refs/heads/master')])
(12 additional frame(s) were not displayed)
...
  File "swh/scrubber/storage_checker.py", line 96, in _check_postgresql
    self.process_objects(objects)
  File "swh/scrubber/storage_checker.py", line 100, in process_objects
    real_id = object_.compute_hash()
  File "swh/model/model.py", line 221, in compute_hash
    return self._compute_hash_from_attributes()
  File "swh/model/model.py", line 743, in _compute_hash_from_attributes
    return _compute_hash_from_manifest(git_objects.snapshot_git_object(self))
  File "swh/model/git_objects.py", line 505, in snapshot_git_object
    unresolved,

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Migrated from T4425 (view on Phabricator)