Comments imported from a remote bug tracker should be committed before we try to push comments

[Graham Binns]

At the moment, the workflow for syncing comments goes like this:

 1. Begin transaction
 2. Pull comments
 3. Push comments
 4. Commit transaction

If an error is raised at either point 2 or point 3, the transaction is immediately aborted. This is okay if an error occurs when pulling comments, but if we've already successfully imported some comments and then encounter an error when pushing comments, the comments we've pulled in are lost when the transaction is rolled back.

The workflow should be:

 1. Begin transaction
 2. Pull comments
 3. Commit transaction
 4. Begin transaction
 5. Push comments
 6. Commit transaction