When users amend commit messages and then run "arc diff" without --verbatim, their changes are silently discarded.

Instead, we should detect they've changed the commit message (store a hash after amending it? How do we deal with a stack of messages? Store hashes of all of them?), and ideally detect how they've changed it (if they changed "Summary", we can recommend "--verbatim"; if they changed "CC", we can recommend "--edit"). Specifically, the three supported ways to update revisions are:

• Edit from the web UI.
• Use "arc diff --edit".
• Amend some fields locally and use "arc diff --verbatim".

Possibly we could just dump a comment into the non-final form of commit messages, like:

# DO NOT EDIT THIS MESSAGE
# Use "arc diff --edit" to edit revision information.

However, this is gross.