Merge branch 'cw/rebase-i-root'

Finishing touches to the "rebase -i --root" (new feature for
1.7.12).

* cw/rebase-i-root:
  rebase -i: handle fixup of root commit correctly

diff --cc git-rebase--interactive.sh
Simple merge

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