R1:2d9d2f1aede1

Allow "Differential Revision ID" field appear on create/edit messages

Summary:
- In practice, 'edit' has two modes, 'create' and 'edit'. These seem like they should map to "create a revision" and "update a revision", but they are completely different.
- We use the "create" mode:
- When creating a message from the working copy.
- When creating a message from a file.
- When creating a message from a commit.
- When creating a message from a user template.
- When creating a…