When delegates collaborate on the repository identity, their voting
narrows the tree of active proposals to the chain of accepted
proposals. Thus, they crucially must understand which proposals are
siblings, i.e., which proposals share the same parent, because no
two sibling proposals should ever be accepted at the same time, as
this would mean a fork.
To ease collaboration of delegates and improve their overview, also
print the parent revision, if present. Note that the initial revision
is the only revision that has no parent.
998ff91e2c