The underlying `Dag` for a COB will use the `Ord` implementation of
whatever is provided as the key. This means that the lexicographical
ordering of the `Oid` SHA will be used, which in turn means that if
the SHAs change in our tests, then the ordering will be broken for
concurrent updates.
To prevent this from happening, a variant of the `prune` method is
introduced that takes an ordering of the keys and values – this method
being called `prune_by`.
This allows `prune` to be replaced with `prune_by` in the
`ChangeGraph::evaluate` method. The ordering used is first comparing
the `Entry::timestamp` and then the `EntryId` (`Oid`).
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