Allow using git rebase -i --root

[MateuszKubuszok]

If one tries to redate all commits including the initial commit, git-redate would fail (because HEAD~n whould point before the original commit). Newer git versions allow using --root as a commit. It would be nice if git-redate allow that.

[danielthedifficult]

Having this same issue

[nagarajpandith]

same issue here.