copies: add a new test dedicated to testing chain of changeset with merge The copies test we currently have usually focus on simple case that do not dive too much into longer chains involving merges. This new test file focus on extensive testing of these case to validate their behavior and make sure the various copies algorithm have the same behavior. And… actually these test are currently broken for the changeset centric algorithm since 99ebde4fec99, but it went undetected because these case were not tested. Differential Revision: https://phab.mercurial-scm.org/D8078