R1:64509dcca7be

Drive CLI-based revision edits through "differential.revision.edit" API + EditEngine

Summary:
Ref T11114. This creates `differential.revision.edit` (a modern, v3 API method) and redefines the existing methods in terms of it.

Both `differential.createrevision` and `differential.updaterevision` are now internally implemented by building a `differential.revision.edit` API call and then executing it.

I //think// this covers everything except custom fields, which need some tweaking to work with EditEngine.