verify: pass a revlog to `_checkrevlog` in `_verifymanifest`
Since `manifestrevlog` is not a `revlog`, we are passing strange thing to
`_checkrevlog`. We fix this to avoid breakage during future change.
Differential Revision: https://phab.mercurial-scm.org/D10564
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}