See IRC at <https://secure.phabricator.com/chatlog/channel/6/?at=110994>. If you `arc diff` in a working copy with changes but no commits, the workflow picks `shouldAmend = false` because there are no commits, and does not amend the commit message after the editor prompt.
Instead, the arc-generated commit should be considered amendable.
Generally, this workflow is really confusing. This is a "bad" workflow (not committing, in the IRC case not using a local branch) but it should be accommodated more gracefully.