trydiff: call util.binary in only one place It's practically free to call util.binary on empty or None content. By relying on that, we can replace the current four call sites by one.

trydiff: collect all lossiness checks in one place By having all the checks for lossiness in one place, it becomes much easier to get an overview of the conditions that lead to losedatafn() being called. It also makes it obvious that it can not be called multiple times for a single time (something that was rather tricky to determine before).