copies-rust: move the parent token to an enum
We carry around information about which parent of a revision is been dealt
with. So far this was a `usize` but as we are about to pass it around to more
function it seems like a good idea to start cleaning this up and use a proper
enum.
Differential Revision: https://phab.mercurial-scm.org/D9420
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}