Merge branch 'cc/merge-base-many'

* cc/merge-base-many:
  git-merge-octopus: use (merge-base A (merge B C D E...)) for stepwise merge
  merge-base-many: add trivial tests based on the documentation
  documentation: merge-base: explain "git merge-base" with more than 2 args
  merge-base: teach "git merge-base" to drive underlying merge_bases_many()