copies: move early return in mergecopies() earlier 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. Differential Revision: https://phab.mercurial-scm.org/D7975

tests: test merge of renames of different sources to same target This is a really obscure scenario, but let's still have it tested so we know when it changes. Differential Revision: https://phab.mercurial-scm.org/D7985