revlog: make calls to _isgooddelta() consistent We always want to call _isgooddelta() before accepting a delta. We mostly call the function right after building the delta, but not always. Instead, we have an extra call at the end of the big code block. Let's make it consistent, so we call _isgooddelta() right after builddelta() and exactly once per delta. That also lets us rely on "delta is None" to mean we didn't find a good delta.