Merge branch 'ds/merge-base-independent'

The code to implement "git merge-base --independent" was poorly done and was kept from the very beginning of the feature. * ds/merge-base-independent: commit-reach: stale commits may prune generation further commit-reach: use heuristic in remove_redundant() commit-reach: move compare_commits_by_gen commit-reach: use one walk in remove_redundant() commit-reach: reduce requirements for remove_redundant()