static-http: have `statichttprepo.instance` return a peer object
It previously returned a statichttprepo object which could not be used for
anything but creating a peer. So lets put it into the peer bucket with a peer
behavior.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}