Add way to abort delta generation on matching hash

[Plootie]

I notice that FastRsync performs a hash of the entire file whenever it creates a signature or builds a delta, however even if these hashes match each other in both algorithm and value the delta builder will still run over the entire stream.

It would be amazing to have some sort of way to either perform the metadata creation and delta generation separately to allow comparison or have the delta builder skip going over the stream if the hashes match