stream-clone: add the `-exp` prefix to the bundle part
We forget to do so in 58adcabc295f, however this is important to prevent
"current" client to send incompatible version to future client.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}