improved default action optimization

prev impl was relying on the correct order, which wasn't guaranteed
formatting
order actions by size (asc)

quick wins
wording to make it more clear

this method is REALLY important. since we'll support cancellation at any
point, we need atomic overwrites (delete + add) operations, so we don't
leave the destination in a bad state. this is also important when
sorting by size.
replaced (the awesome) ListDiff

ListDiff is too sophisticated for this ;)