rank: naive rank property computation and retrieval This stores the rank (size of the ancestor set of a revision, including itself) in a changelog field and allows this property to be retrieved. This new property is used as part of stable-range computations, which will be introduced later on. The value is computed in a naive way from the definition of the rank. This will be replaced by a more efficient version subsequently. Differential Revision: https://phab.mercurial-scm.org/D12139