streamclone: rework canperformstreamclone
There is code about bundle2 laying around in `canperformstreamclone` but not
put to any uses. As we discovered with the previous patch, streambundle 'v1'
won't work on bundle2 because they are readline based. So we jump to 'v2' as
the first expected supported version.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}