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

fivestars/arcanist,Symplicity/arcanist,alexmv/arcanist,coursera/arcanist,coursera/arcanist,kevinseelbach/arcanist,chundongwang/arcanist,Khan/arcanist,eSpark/arcanist,haskell-infra/arcanist,folsom-labs/arcanist,Symplicity/arcanist,alexmv/arcanist,r4nt/arcanist,r4nt/arcanist,Khan/arcanist,fivestars/arcanist,zhihu/arcanist,haskell-infra/arcanist,phacility/arcanist,folsom-labs/arcanist,calblueprint/arcanist,wharris/arcanist,wxstars/arcanist,haskell-infra/arcanist,hach-que/arcanist,jra3/arcanist,calblueprint/arcanist,KieranHayes/arcanist,PageUpPeopleOrg/arcanist-patches,zhihu/arcanist,kevinseelbach/arcanist,sneakpeeq/arcanist,jra3/arcanist,wxstars/arcanist,wikimedia/phabricator-arcanist,wxstars/arcanist,sneakpeeq/arcanist,calblueprint/arcanist,fivestars/arcanist,memsql/arcanist,coursera/arcanist,kevinseelbach/arcanist,kevinseelbach/arcanist,fivestars/arcanist,PageUpPeopleOrg/arcanist-patches,uber/arcanist,PageUpPeopleOrg/arcanist-patches,folsom-labs/arcanist,codelabs-ch/arcanist,chundongwang/arcanist,r4nt/arcanist,memsql/arcanist,Symplicity/arcanist,aik099/arcanist,uber/arcanist,wxstars/arcanist,codelabs-ch/arcanist,alexmv/arcanist,zhihu/arcanist,alexmv/arcanist,hach-que/arcanist,eSpark/arcanist,eSpark/arcanist,wikimedia/phabricator-arcanist,jra3/arcanist,coursera/arcanist,phacility/arcanist,chundongwang/arcanist,codelabs-ch/arcanist,KieranHayes/arcanist,aik099/arcanist,wharris/arcanist,aik099/arcanist,aik099/arcanist,Symplicity/arcanist,chundongwang/arcanist,phacility/arcanist,memsql/arcanist,calblueprint/arcanist,codelabs-ch/arcanist,phacility/arcanist,wikimedia/phabricator-arcanist,Khan/arcanist,folsom-labs/arcanist,uber/arcanist,sneakpeeq/arcanist,codelabs-ch/arcanist,r4nt/arcanist,wharris/arcanist,phacility/arcanist,KieranHayes/arcanist,jra3/arcanist,PageUpPeopleOrg/arcanist-patches,eSpark/arcanist,zhihu/arcanist,memsql/arcanist,KieranHayes/arcanist,wikimedia/phabricator-arcanist,sneakpeeq/arcanist,wharris/arcanist,hach-que/arcanist,haskell-infra/arcanist,Khan/arcanist,hach-que/arcanist,uber/arcanist