folder_branch_ops: fix latest merged rev when journal is empty (#453)
folder_branch_ops: fix latest merged rev when journal is empty (#453)

We need to use the server's notion of the merged head when the journal
is empty.
keybase/client,keybase/client,keybase/kbfs,keybase/kbfs,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client