Mark arc svn-hook-pre-commit for subversion only

Summary: It doesn't make any sense to run this command on any other VCS.

Test Plan: Ran `arc svn-hook-pre-commit` and hit the usage exception.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11623

src/workflow/ArcanistSvnHookPreCommitWorkflow.php

@@ -36,6 +36,10 @@ EOTEXT
     return false;
   }
 
+  public function getSupportedRevisionControlSystems() {
+    return array('svn');
+  }
+
   public function run() {
     $svnargs = $this->getArgument('svnargs');
     $repository = $svnargs[0];