mercurial.loader: Migrate to swh.model.hashutil.MultiHash

Note that i potentially broke silently (due to no test T.T) when migrating swh-model (~> removing the with_length from the hashutil.hash_data function)

That also fixes it (and computes the length of the data one less time).

Related swh-model!12 (closed)

Test Plan

Manual

Following the readme samples [1] to execute the loading on repositories.

• [1] https://forge.softwareheritage.org/D437#change-e9wbZ9Ppa6Oy

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Migrated from D437 (view on Phabricator)