Previously, we'd use the current head if not otherwise specified, even
when the user specified a epoch. Now:
1. If the user specifies nothing, we use head head's epoch.
2. If the user specifies a tipset and no epoch, we use that tipset and
the epoch of that tipset.
3. If the user specifies an epoch and no tipset, use the tipset at that
epoch (based on the current head).
4. Finally, if the user species both, use both (allowing the
epoch/tipset to disagree).
Steven Allen
dfd3b10182