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

changelog-v2: fix an assertion error to display the right data We were checking on thing and displaying the other. Differential Revision: https://phab.mercurial-scm.org/D11935

changelog-v2: use helper constant in the code to pack/unpack entries This makes the code simpler to read and modify. This is probably less efficient, but we are talking about the Python implementation here, so performance is concerne secondary to readability. Differential Revision: https://phab.mercurial-scm.org/D11934