R1:e7fde9a77c49

Make repository discovery partially testable

Summary:
Ref T2784. Begins pulling discovery into Engines and covering it with tests. In particular:

- Discovery is currently a one-shot process where we find all the new commits and write them to the database in one go. Split it apart so we find and return the new commits first, then write them to the database separately. This makes things simpler and more testable.
- This diff only brings SVN into an engine (and only the "find the commits" part), since…