manifest: add treemanifestctx class Before we start using repo.manifestlog in the rest of the code base, we need to make sure that it's capable of returning treemanifests. As we add new functionality to manifestctx, we'll add it to treemanifestctx at the same time. We also comment out the manifestctx p1, p2, and linkrev fields for now, since we're not implementing them on treemanifest yet.