manifest: move dirlog up to manifestrevlog This removes dirlog and its associated cache from manifest and puts it in manifestrevlog. The notion of there being sub-logs is specific to the revlog implementation, and therefore belongs on the revlog class. This patch will enable future patches to move the serialization logic for manifests onto manifestrevlog, which will allow us to move manifest.add onto manifestlog in a way that it just calls out to manifestrevlog for the serialization.