R1:912509558723

Distinguish between empty and unparsed commits in Diffusion

Summary:
Fixes T3416. Fixes T1733.

- Adds a flag to the commit table showing whether or not we have parsed it.
- The flag is set to `0` initially when the commit is discovered.
- The flag is set to `1` when the changes are parsed.
- The UI can now use the flag to distinguish between "empty commit" and "commit which we haven't imported changes for yet".
- Simplify rendering code a little bit.
- Fix an issue with the Message parser for…