Fix a bad interaction between "arc diff --reviewers" and "the first line of a message is always a title"

Summary:
Fixes T12069. We implement "arc diff --reviewers" (and "--cc") by parsing a faux message with "Reviewers: ...".

After D17122, the first line of the message is always interpreted as a title, so the text ends up in the message body.

Instead, use a placeholder title so these fields are never initial fields.

Test Plan: Ran arc diff --reviewers dog, got only one "Reviewers" field.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12069

Differential Revision: https://secure.phabricator.com/D17147