Forward Git SSH sessions from gitlab-shell to Gitaly Server Implementation

Depends on Migration Analysis #91 (closed)

Depends on RPC Design gitaly-proto!5 (merged)

Server Implementation Forward Git SSH sessions from gitlab-shell to Gitaly

Goal: allow Gitaly to monitor and control the git upload-pack and git receive-pack server-side processes spawned by the Git SSH transport.

We already have a protocol for this. https://gitlab.com/gitlab-org/gitaly-proto/blob/v0.5.0/ssh.proto

We want to be able to run two commands on behalf of SSH clients: git upload-pack and git receive-pack. Each has its own RPC SSHUploadPack and SSHReceivePack.

The idea behind the current proto is:

read first request message to get Repository and some metadata (GL_ID variable) in case of receive-pack
start the git process requested for the correct git repository path
in a goroutine: copy the rest of the request stream to stdin
in two goroutines: copy the stdout and stderr streams to the response
wait for the git command, out and err streams to finish
send final response message containing the git exit code

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