Merge branch 'pw/rebase-i-show-HEAD-to-reword'

"git rebase -i" showed a wrong HEAD while "reword" open the editor.

* pw/rebase-i-show-HEAD-to-reword:
  sequencer: simplify root commit creation
  rebase -i: check for updated todo after squash and reword
  rebase -i: always update HEAD before rewording

diff --cc sequencer.c
Simple merge

diff --cc t/t3404-rebase-interactive.sh
Simple merge