git rebaseでinitial commitをまとめる

gitコマンドのTIPSです。
2個だけコミットがあり、その２つをrebaseでまとめたい時、git rabese -i HEAD^^を実行してもinitial commmitが表示されません。
代わりに以下のコマンドを発行すると、ちゃんとinitial commitもrebaseできます。

$ git rebase -i --root

pick 8ffb8da first commit
pick bda2f1d test

# Rebase bda2f1d onto c38fdf9 (2 command(s))
#
# Commands:
# p, pick = use commit
# r, reword = use commit, but edit the commit message
# e, edit = use commit, but stop for amending
# s, squash = use commit, but meld into previous commit
# f, fixup = like "squash", but discard this commit's log message
# x, exec = run command (the rest of the line) using shell
# d, drop = remove commit
#
# These lines can be re-ordered; they are executed from top to bottom.
#
# If you remove a line here THAT COMMIT WILL BE LOST.
#
# However, if you remove everything, the rebase will be aborted.
#
# Note that empty commits are commented out