fix long-standing bug in syncer.chunk()

chunk() was failing to gather anything but the first batch
of changes as the logic to determine whether there were
more changes was wrong.

with the recent change to a 250 chunk size, any users
who changed more than 250 cards on AD and synced may have
collections that don't match what's on AnkiWeb, so
for the 2.3.1 release I recommend you advise  users to force
a full sync to correct any possible problems
Merge pull request #601 from dae/sync-fix2

fix long-standing bug in syncer.chunk()