copies: directly pass a changes object to the copy tracing code The object contains all the data we need. For example, the `is_merged` callback can now use the associated precomputed data. This will be useful again soon when the `salvaged` set will be introduce to solve the issue with delete file reverted during a merge. See 4b582a93316a and 14be07d5603c for details. Differential Revision: https://phab.mercurial-scm.org/D9117