Add feature to check if a persisted graph is different than what was sent

[cbeer]

e.g. revert 88b13cbc2cdc08ac830bf0530aeb3d882440a362

[no-reply]

This might be a use case for RDF::Transaction.

We'd have to add a #changed? API, but that seems like a nice feature add if we could get it right. The implementation on the default Repository::Implementation would be a cheap object equality check.

Obviously, this would require a refactor to use the Transaction throughout.