graph: spin-off rank-related methods into RankedGraph
Making a clear distinction between graphs with cheap rank queries and those which require more intensive computations.
This follows review comment mercurial-devel!97 (diffs, comment 166948)
Making a clear distinction between graphs with cheap rank queries and those which require more intensive computations.
This follows review comment mercurial-devel!97 (diffs, comment 166948)