rank: add a "rank" value to the revlog-entry tuple 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. Differential Revision: https://phab.mercurial-scm.org/D11936

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