It is a little bit scuffed, but it seems to produce correct results. We
can run it at a later date when we want to explode every in-flight
commit in existence and then need to filter-branch them.
Fixes: https://git.lix.systems/lix-project/lix/issues/188
Change-Id: Id97e4651f78804a941d941df02c7c1b21ce453b6