Differential - fix bug writing affected paths

Summary: Fixes T4774. With the new code and configuration instructions downplaying the role of arcanist project we weren't writing affected paths at all! I had this issue on my installation - no affected paths were written. We seem to always have the repository now though if we can see it, so not too bad of a fix.

Test Plan: updated a diff and was able to browse in diffusion.

Reviewers: epriestley, bitglue

Reviewed By: epriestley, bitglue

Subscribers: bitglue, epriestley, Korvin

Maniphest Tasks: T4774

Differential Revision: https://secure.phabricator.com/D8757