Normalize Windows directory separators in SVN

Summary: Fixes T1946.

Test Plan: Ran arc diff --only with added paths inside directories. Saw only "/" entries in the diff.

Reviewers: btrahan, vrana

Reviewed By: vrana

CC: aran, mbishopim3

Maniphest Tasks: T1946

Differential Revision: https://secure.phabricator.com/D4374