Previously, we would fetch each file in the revision/changeset
individually. With this change, we fetch the entire revision in one
request, and use a custom editor to verify its contents. This is quite
a lot faster than the previous means when verifying over the internet.
By an order of magnitude or two, in fact. As data is transfered in a
single operation, verifying a revision from PyPy took 30 seconds
rather than 30 minutes, and saturated my 10Mbps connection.
Please note that the output ordering isn't stable between the two;
output will appear in reverse order when using the fast verifier.
