R1:220addb249fe

Move Git discovery into DiscoveryEngine

Summary:
Ref T4327. Consolidates and simplifies infrastructure:

- Moves Git discovery into DiscoveryEngine.
- Collapses a bunch of the Git and Mercurial code related to stream discovery.
- Removes all cach code from PullLocal daemon (it's no longer called).
- Adds basic unit tests for Git and Mercurial discovery.

Various cleanup:

- Makes GitStream and MercurialStream extend a common base.
- Improves performance of MercurialStream in some cases, by…