Merge branch 'js/rebase-cleanup'

Merge branch 'js/rebase-cleanup'

Update supporting parts of "git rebase" to remove code that should
no longer be used.

* js/rebase-cleanup:
  rebase: fold git-rebase--common into the -p backend
  sequencer: the `am` and `rebase--interactive` scripts are gone
  .gitignore: there is no longer a built-in `git-rebase--interactive`
  t3400: stop referring to the scripted rebase
  Drop unused git-rebase--am.sh