folder_branch_ops: fix setHead when there are no flushed updates (#446)
folder_branch_ops: fix setHead when there are no flushed updates (#446)

Suggested by @akalin-keybase.

Issue: #444
keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/kbfs,keybase/client