SSHD should pass through GIT_PROTOCOL env var
In order to support git wire protocol v2, SSHD should pass through the client provided environment variable GIT_PROTOCOL. This will in turn be sourced by gitlab-shell and then be passed onto Gitaly where it can toggle the v2 feature.
The SSHD configuration needs to have the following directive added:
AcceptEnv GIT_PROTOCOL
Relates to gitaly#2474 (comment 291552130)