Set GIT_SSH_VARIANT alongside GIT_SSH

A recent version of Git has changed some piece of behavior here and we
now get "fatal: ssh variant 'simple' does not support setting port"
when using a port. Explicitly setting GIT_SSH_VARIANT to `ssh` likely
fixes this.

src/applications/diffusion/protocol/DiffusionGitCommandEngine.php

@@ -27,6 +27,7 @@ final class DiffusionGitCommandEngine
     $env['HOME'] = PhabricatorEnv::getEmptyCWD();
 
     $env['GIT_SSH'] = $this->getSSHWrapper();
+    $env['GIT_SSH_VARIANT'] = 'ssh';
 
     if ($this->isAnyHTTPProtocol()) {
       $uri = $this->getURI();