manifest: add manifestlog.add This adds a simple add() function to manifestlog. This lets us convert more uses of repo.manifest to use repo.manifestlog, so we can further break our dependency on the manifest class.

manifest: move manifest.add onto manifestrevlog This moves add and _addtree onto manifestrevlog. manifestrevlog is responsible for all serialization decisions, so therefore the add function should live on it. This will allow us to call add() from manifestlog, which lets us further break our dependency on manifest.

manifest: remove dependency on treeinmem from manifest.add Currently manifest.add uses the treeinmem option to know if it can call fastdelta on the given manifest instance. In a future patch we will be moving add() to be on the manifestrevlog, so it won't have access to the treeinmem option anymore. Instead, let's have it actually check if the given manifest instance supports the fastdelta operation. This also means that if treemanifest or any implementation eventually implements fastdelta(), it will automatically benefit from this code path.