R1:95fb237ab393

On Git cluster read failure, retry safe requests

Summary:
Depends on D20775. Ref T13286. When a Git read request fails against a cluster and there are other nodes we could safely try, try more nodes.

We DO NOT retry the request if:

- the client read anything;
- the client wrote anything;
- or we've already retried several times.

Although //some// requests where bytes went over the wire in either direction may be safe to retry, they're rare in practice under Git, and we'd need to puzzle out what…