obsolete: add an explicit '_succs.copy()' method
Mimic the standard API for copying in the _succs class, it makes the code
slightly cleaner and will be needed later for copying markers at the same time
than copying the list content.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}