commands: pass through GIT_PROTOCOL envvar provided by clients
Both git-upload-pack and git-receive-pack services inspect the GIT_PROTOCOL environment transferred via SSH in order to decide which protocols are supported by a given client. Currently, we don't use the environment variable at all, though, but instead forward the GitProtocol field of the access verification response.
Improve this by passing on the GIT_PROTOCOL environment variable provided by the client as-is.
Fixes #273 (closed)