Merge branch 'en/rebase-backend'

Band-aid fixes for two fallouts from switching the default "rebase"
backend.

* en/rebase-backend:
  git-rebase.txt: highlight backend differences with commit rewording
  sequencer: clear state upon dropping a become-empty commit
  i18n: unmark a message in rebase.c

diff --cc builtin/rebase.c
Simple merge

diff --cc sequencer.c
Simple merge