[BUG?] rebase -i -p leaves index changed

Hi,

'rebase -i -p' appears to be a bit confused about what it is doing. Try a history like this:

  O -- A -- M -- B
   \       /
    \- C -/

built by:

  git init
  touch foo
  git add foo
  git ci -m initial
  git tag root
  git co -b side
  echo blah > other
  git add other
  git ci -m other
  git co master
  echo a >> foo
  git add foo
  git ci -m a
  git add foo
  git merge side
  echo b >> foo
  git add foo
  git ci -m 'b: depends on side'

First try this:

  GIT_EDITOR=true git rebase -i -p root
  git diff --cached

resulting in

  diff --git a/foo b/foo
  index 422c2b7..e69de29 100644
  --- a/foo
  +++ b/foo
  @@ -1,2 +0,0 @@
  -a
  -b
  diff --git a/other b/other
  deleted file mode 100644
  index 907b308..0000000
  --- a/other
  +++ /dev/null
  @@ -1 +0,0 @@
  -blah

Second, 'edit' is also a bit suspicious:

  git reset --hard
  GIT_EDITOR='perl -i -pe "s/pick 096/edit 096/"' git rebase -i -p root

Despite claiming "Stopped at 096[...] a", a quick 'git show' tells us that it is actually stuck at 'initial'. (At least 'git rebase --continue' then ends up with the same result as the first test.)

Granted, I'm not entirely sure what it _should_ do. In my use case (relating to the filter-branch topic), C was a commit from elsewhere that B depended on. So I kind of hoped 'rebase -i -p' would let me edit A, then rebuild M and B on top, while leaving C alone.

- Thomas