copies: define a type to return from mergecopies() We'll soon return two instances of many of the dicts from `copies.mergecopies()`. That will mean that we need to return 9 different dicts, which is clearly not manageable. This patch instead encapsulates the 4 dicts we'll duplicate in a new type. For now, we still just return one instance of it (plus the separate `diverge` dict). Differential Revision: https://phab.mercurial-scm.org/D7989