Merge branch 'jn/update-contrib-example-merge' into next

Merge branch 'jn/update-contrib-example-merge' into next

* jn/update-contrib-example-merge: (24 commits)
  merge script: learn --[no-]rerere-autoupdate
  merge script: notice @{-1} shorthand
  merge script: handle --no-ff --no-commit correctly
  merge script: --ff-only to disallow true merge
  merge script: handle many-way octopus
  merge script: handle -m --log correctly
  merge script: forbid merge -s index
  merge script: allow custom strategies
  merge script: merge -X<option>
  merge script: improve log message subject
  merge script: refuse to merge during merge
  merge script: tweak unmerged files message to match builtin
  merge script: --squash, --ff from unborn branch are errors
  fmt-merge-msg -m to override merge title
  merge-base --independent to print reduced parent list in a merge
  merge-base --octopus to mimic show-branch --merge-base
  Documentation: add a SEE ALSO section for merge-base
  t6200 (fmt-merge-msg): style nitpicks
  t6010 (merge-base): modernize style
  t7600 (merge): test merge from branch yet to be born
  ...