revlog: add a `canonical_index_file` attribute on inner revlog
This is currently the same and the index_file but it will become more complex
when we start doing delayed write.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}