manifestctx: add _revlog() function The `self._repo.manifestlog._revlog` code is getting copy and pasted a lot in manifestctx. Let's make it a function so it can be reused. This will make future patches cleaner too.

manifest: remove manifest.find As part of removing dependencies on manifest, this drops the find function and fixes up the two existing callers to use the equivalent apis on manifestctx.

changegroup: remove remaining uses of repo.manifest The remaining uses of repo.manifest in the changegroup module are treating the manifest exclusively as a revlog, so let's replace them with instances of the revlog directly. This is part of dropping all dependencies on repo.manifest in favor of repo.manifestlog.