Rebase does not support fixup workflow

[JakeWharton]

I shouldn't have to do a whole mid-stack edit dance to amend a mid-stack commit. If I do --fixup commits, I would expect rebase to have the equivalent behavior of --autosquash.

Doing a normal git rebase with

[rebase]
	updateRefs = true
	autosquash = true

still works and updates the whole stack correctly, at least.