Fetch SVN property changes for Diffusion

Summary: Fixes T4255.  Previously only the file content was fetched and as such no properties were filled out in Diffusion.  This fetches old and new properties and adds them to the Arcanist diff object for later display.

Test Plan: I did not see where to locate a test for this.  If needed, I'll ask for a quick hand pointing me in the right direction.  Thanks.

Reviewers: epriestley

CC: