RFC: git rebase -i and root commits

My normal workflow is to create a .gitignore in my initial commit.

When I later realize that I've forgotten something from that file
I could of course just commit the changes, but I'd rather use "git rebase -i"
in the normal way to make myself appear smarter than I am.
Especially since this realization usually comes early on
(and certainly before publishing).

But rebase can't go all the way to a root ("fatal: Needed a single revision").
The best I've found is:

1) git checkout -b temp <root commit>
2) echo '*.tmp' >> .gitignore
3) git commit --amend .gitignore
4) git rebase --onto temp <root commit> master

If there really is an asymmetry here and I haven't missed anything
(a large assumption), then what's the best way to think about it?

Is there a role for a default commit (e.g. the sha1 of "") here so that
'rebase -i' can update all commits and not just the ones with parents?

Or should I just get used to it and move on?

Thanks - Eric