R1:d87c500002d7

Synchronize (hosted, clustered, Git) repositories over Conduit + HTTP

Summary:
Ref T4292. We currently synchronize hosted, clustered, Git repositories when we receive an SSH pull or push.

Additionally:

- Synchronize before HTTP reads and writes.
- Synchronize reads before Conduit requests.

We could relax Conduit eventually and allow Diffusion to say "it's OK to give me stale data".

We could also redirect some set of these actions to just go to the up-to-date host instead of connecting to a random…