We have such a test already (squash_fixups_above_first_commit.go), but it can't
be used for what we want to check here, because it uses the first commit, and we
can't move down from there. So create a new one that basically does the same
thing, but for a commit in the middle. The focus of this new test is to check
how the selection behaves; as you can see, there is a problem both when creating
a fixup and when squashing fixups. We'll address these separately in the next
commits.