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
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