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

mercurial/verify.py

@@ -326,7 +326,7 b' class verifier(object):'
         if self.refersmf:
             # Do not check manifest if there are only changelog entries with
             # null manifests.
-            self._checkrevlog(mf, label, 0)
+            self._checkrevlog(mf._revlog, label, 0)
         progress = ui.makeprogress(
             _(b'checking'), unit=_(b'manifests'), total=len(mf)
         )