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
Differential Revision: https://phab.mercurial-scm.org/D8078