If the user runs git log while rewording a commit it is confusing if
sometimes we're amending the commit that's being reworded and at other
times we're creating a new commit depending on whether we could
fast-forward or not[1]. For this reason the reword command ensures
that there are no uncommitted changes when rewording. The reword
command also allows the user to edit the todo list while the rebase is
paused. As 'merge -c' also rewords commits make it behave like reword
and add a test.