I think any rewrite should be explicitly done by the user. I'm not all that familiar with this code, but it looks like the user gets to return the whole set, in which case they should run the text through the prettify function if they want to  (e.g. the equivalent of the 'reword' option in `git-rebase--interactive` would likely want to do this, as the user was asked for input), but we shouldn't change the data the user told us to put into the commit.