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 empty commits.
- There's a key on the flag so we can do SELECT * FROM repository_commit WHERE repositoryID = %d AND importStatus = 0 LIMIT 1 soon, to determine if a repository is fully imported or not. This will let us improve the UI (Ref T776, Ref T3217).