[svn1.7] Fix matching xml tag when parsing svn commits

Summary:
svn 1.7 changed their xml format slightly, they now have a
<?xml version="1.0" encoding="UTF-8"?> tag instead of
<?xml version="1.0"?>. This relaxes matching this tag.

Test Plan: ./scripts/repository/reparse.php rE521979 --change

Reviewers: epriestley, jungejason

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1866