folder_branch_ops: use cached direntries for GetDirChildren
folder_branch_ops: use cached direntries for GetDirChildren

Issue: KBFS-568

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