Fix `arc diff x y` in SVN
Fix `arc diff x y` in SVN

Summary:
D4186 added an "svn status --xml x y" form to getSVNStatus(), but the parser doesn't work for multiple files, since we get multiple <target /> elements in the XML output. So, curently, `arc diff` works (one target, all files) and `arc diff x` works (one target, x) but `arc diff x y` does not (more than one target, hits the exception).

  $ arc diff QUACK2 QUACK3
  Exception
  Expected exactly one XML status target.

Test Plan: Ran `arc diff QUACK2 QUACK3` in a working copy with modified QUACK2, QUACK3. Ran `arc diff`; `arc diff QUACK2`.

Reviewers: vrana, btrahan, codeblock, JThramer

Reviewed By: codeblock

CC: aran

Differential Revision: https://secure.phabricator.com/D4372

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