Merge branch 'ds/reachable-topo-order'

The revision walker machinery learned to take advantage of the commit generation numbers stored in the commit-graph file. * ds/reachable-topo-order: t6012: make rev-list tests more interesting revision.c: generation-based topo-order algorithm commit/revisions: bookkeeping before refactoring revision.c: begin refactoring --topo-order logic test-reach: add rev-list tests test-reach: add run_three_modes method prio-queue: add 'peek' operation