manifest: drop the indexfile from manifestrevlog
Since manifestrevlog object are not revlog (no really, they are not…) we drop
the revlog specific attribute. We need to directly access the underlying revlog
in a couple of place that already assume that we have a revlog here.
This is motivated by future change to that revlog attribute.
Differential Revision: https://phab.mercurial-scm.org/D10572