The --diff-only flag has not really worked correctly in a while. Its

purpose was to only post a diff for review, ignoring anything content in

a Perforce change description, and it didn't do anything at all for

non-Perforce repositories. It just controlled whether we set a change

number server-side, which would cause Review Board to pull down a new

change description from Perforce.
It was originally orthogonal to review request guessing, but recently

Perforce received guessing support. Normally, with default settings,

guessing only happens when not passing -u or -r, but with Perforce,

you don't pass either of these. You just pass a change number. So,

guessing would kick in and cause a summary and description to be

provided. It also unconditionally caused the changenum field to also

be provided, which triggered the server-side updating.
This change restores the original intent of --diff-only, and makes it

universal. Now, if specified, only the diff will be uploaded. All fields

(except for public, if --publish is specified) will be left out of

any payload. It disables guessing, disables fields specified on the

command line, and disables commit IDs/change numbers.
This should restore the expected behavior on Perforce and provide

benefits to those on other types of repositories.

Tested with --diff-only and verified that only the diff information

was provided in the payload, and that guessed fields and the change

number/commit ID were not.
This was tested (along with --change-only) on both Perforce and Git.