From IRC:

<csilvers>: epriestly: another 'arc diff --verbatim' bug: if I do 'arc diff --verbatim' on an existing review, and there's been no new git commits since the last 'arc diff --verbatim' (because I used arc amend), then the editor that pops up says: "Updating D: ...." instead of "Updating D1234: ..." or whatnot