folder_branch_ops: set the Writer before creating the syncOp
folder_branch_ops: set the Writer before creating the syncOp

Otherwise, later AddUpdate calls won't find the right BlockPointer and
won't populate the syncOp.File field properly.

keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/kbfs