To support this, we turn the confirmation prompt of the "Create fixup commit"
command into a menu; creating a fixup commit is the first entry, so that
"shift-F, enter" behaves the same as before. But there are additional entries
for creating "amend!" commits, either with or without file changes. These make
it easy to reword commit messages of existing commits.
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.
