revlog: move the `rawtext` method on the inner object This is one of the current core goal of the inner object: having a "simple" API for some of the core revlog action, like getting a raw text, with the inner object able to do that part internally, on its own.

{
    "conduit_uri": "https://phab.mercurial-scm.org/api",
    "phabricator.uri": "https://phab.mercurial-scm.org/",
    "repository.callsign": "HG",
    "arc.land.onto.default": "@",
    "base": "hg:.^"
}