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