Allow using Built-in SSH with non-default port on git >= 2.16

Git tries to auto-detect ssh executable type to use correct command line. It detects Built-in SSH as simple, which does not support specifying port.
Git 2.16 started to error out when port is specified in the URL and simple ssh is used.
See https://public-inbox.org/git/20171120213101.yquv2ywyjajjtr5x@aiede.mtv.corp.google.com/

Git can be configured to treat builtin SSH as a regular ssh client with GIT_SSH_VARIANT environment variable set to ssh

Fixes IDEA-185221

plugins/git4idea/rt/src/org/jetbrains/git4idea/ssh/GitSSHHandler.java

@@ -38,6 +38,10 @@ public interface GitSSHHandler {
    * Name of environment variable for SSH executable
    */
   @NonNls String GIT_SSH_ENV = "GIT_SSH";
+  /**
+   * Name of environment variable for SSH executable variant
+   */
+  @NonNls String GIT_SSH_VAR = "GIT_SSH_VARIANT";
   /**
    * Name of the handler
    */

plugins/git4idea/src/git4idea/commands/GitHandlerAuthenticationManager.java

@@ -110,6 +110,7 @@ class GitHandlerAuthenticationManager implements AutoCloseable {
   private void prepareSshAuth() throws IOException {
     GitXmlRpcSshService ssh = ServiceManager.getService(GitXmlRpcSshService.class);
     myHandler.addCustomEnvironmentVariable(GitSSHHandler.GIT_SSH_ENV, ssh.getScriptPath().getPath());
+    myHandler.addCustomEnvironmentVariable(GitSSHHandler.GIT_SSH_VAR, "ssh");
     mySshHandler = ssh.registerHandler(new GitSSHGUIHandler(myProject, myHandler.isIgnoreAuthenticationRequest()), myProject);
     myHandler.addCustomEnvironmentVariable(GitSSHHandler.SSH_HANDLER_ENV, mySshHandler.toString());
     int port = ssh.getXmlRcpPort();