manifest: remove _repo from manifestctx objects We were storing the repo on the manifestctx objects so that they could access the manifestlog via repo.manifestlog, which would refresh the structure if it became out of date. This caused probems however when we want to have multiple manifest logs in memory at once, like when transitioning to tree manifest from flat manifests, since a tree manifest would try to access sub-trees via repo.manifestlog[node], which was the flat manifest. The solution is to just not store the repo, and instead store the manifestlog that created this context. This removes the invalidation when the in memory manifestlog becomes out of date, but people should probably not be keeping ctx's around that long anyway.