Calling total or lastBatch on a batch will issue two COUNT queries

This was discovered during the beta of the new custom bug listings feature work on Launchpad. If you have a batch and call total *and* lastBatch on it, i.e.:

total = batch_nav.batch.total()
last_batch = batch_navigator.batch.lastBatch()

then the batch navigator will issue two COUNT queries. For certain navigation lists, this extra count can be deadly. The code takes some precautions to cache listlength in a local variable, but for some reason, the sample code above forces a second query.