Set GIT_SSH_VARIANT when setting GIT_SSH

Make it explicit that the ssh wrapper we use for control master
support accepts OpenSSH-compatible command line arguments instead of
asking Git to guess.

The GIT_SSH_VARIANT setting was introduced in Git v2.13.0-rc0~3^2~2
(2017-02-01) as a more reliable detection method than relying on the
ssh command name.  Fortunately the default variant was 'ssh' (i.e.,
OpenSSH-compatible) so this wasn't initially required.

Now Git wants to start using more OpenSSH features
(-o SendEnv=GIT_PROTOCOL), and in order to do so its ssh variant
detection will need to be tweaked.  Set GIT_SSH_VARIANT explicitly
so this helper can continue to work regardless of how Git modifies
its autodetection.

Reported-by: William Yan <wyan@google.com>
Change-Id: I6bf2c53b4eb5303a429eae6cb68e0a5ccce89064

git_command.py

@@ -152,6 +152,7 @@ class GitCommand(object):
     if ssh_proxy:
       _setenv(env, 'REPO_SSH_SOCK', ssh_sock())
       _setenv(env, 'GIT_SSH', _ssh_proxy())
+      _setenv(env, 'GIT_SSH_VARIANT', 'ssh')
     if 'http_proxy' in env and 'darwin' == sys.platform:
       s = "'http.proxy=%s'" % (env['http_proxy'],)
       p = env.get('GIT_CONFIG_PARAMETERS')