It wasn't obvious that the early return happened only when there are
no copies. That is the case, however, because if fullcopy is empty,
then so is copies1 and copies2, and then so is inversecopies1
and inversecopies2, and then so is allsources, and then so is
copy, diverge and renamedelete.
By moving the early return earlier, we also avoid calculating the set
of added files from the base to each side.