manifest: remove dependency on manifestrevlog being able to create trees A future patch will be removing the read() function from the manifest class. Since manifestrevlog currently depends on the read function that manifest implements (as a derived class), we need to break the dependency from manifestrevlog to read(). We do this by adding an argument to manifestrevlog.write() which provides it with the ability to read a manifest. This is good in general because it further separates revlog as the storage format from the actual inmemory data structure implementation.