copies: fix some comment in _filter
The scenario the comment describes match case 6, not case 2.
Differential Revision: https://phab.mercurial-scm.org/D9793
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}