copies: simplify merging of copy dicts on merge commits
After we removed some filtering in 35d674a3d5db (copies: don't filter
out copy targets created on other side of merge commit, 2019-04-18),
we will always include all entries from "copies1", so we can simplify
the code based on that.
Differential Revision: https://phab.mercurial-scm.org/D6561