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