Equivalent of "fixup" for git filter-branch

Hello,

I know there is `skip_commit` for git filter branch with
--commit-filter which squash the current commit with the next one,
however I have trouble finding a "fixup" equivalent that'd squash the
commit with the previous one (just like git rebase -i allows).

Yes, I know I could probably use git rebase instead but rebase doesn't
keep the existing commiter date like git filter-branch does.

Philippe