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
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