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
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