Code review comment for lp://staging/~niemeyer/lbox/long-summary

Applied your suggestions on https://codereview.appspot.com/6305112

Except for the behavior on an empty log. This is a well known way to
cancel the action on that kind of software (bzr, git, etc), and I'm keen
on preserving that.

https://codereview.appspot.com/6302100/