The rank of a revision is the size of sub-graph it defines as a head. In other
words, the rank of X is the size of ancestors(X) (X included).
This is a property that can help various algorithm and we intend to store it in
changelog-v2. We start with adding this new information to the "entry tuple",
with a default value. We will start to compute and persist the rank later.
s/fof/of