folder_branch_ops: exit early from SetInitialHeadFromServer
folder_branch_ops: exit early from SetInitialHeadFromServer

If the head is already set, no need to set it again.  This is
especially important in tests that use a staller, as it avoids needing
to take mdWriterLock when simply checking initialization.

Issue: KBFS-1497

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