Mercurial changeset can contain extra data in it, notably a transplant_source field
referencing another changeset id. In contrary to other extra fields, this one stores
the changeset reference in binary instead of using the hexadecimal representation.
So to ease original changeset extra data exploitation and avoid decoding errors,
prefer to store the hexadecimal representation of the changeset id in the
swh revision metadata.
Related T1729