Purge is not called by default any more. We're managing the results of
the generation in a git repo, so don't delete the entire directory every
Compare the list of zones old vs new and delete any old. This allows the
user to rename a zone and not leave files around.
The comparison for SOA record bumping didn't work until the introduction
of the remote vs local concept of zone files. Now, you can point dnsgit
to the old zones on your laptop and it'll only update the zones that
have actually changed.