R1:9b359affe7b5

Prepare SSH connections for proxying

Summary:
Ref T7034.

In a cluster environment, when a user connects with a VCS request over SSH (like `git pull`), the receiving server may need to proxy it to a server which can actually satisfy the request.

In order to proxy the request, we need to know which repository the user is interested in accessing.

Split the SSH workflow into two steps:

# First, identify the repository.
# Then, execute the operation.

In the future, this will allow us to put a possible…