copies: extract an explicit `computechangesetcopie` method from context
Right now, the logic around changeset centric copies data are buried into the
"changectx" code. We extract this code in a dedicated method (in the copies
module) for clarity. This clarity will help to explicitly compute and caches
these data in the future.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}