md_journal: add TODO about MD replacement during branch conversion
md_journal: add TODO about MD replacement during branch conversion

Suggested by @akalin-keybase.

Issue: #395

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