R1:cb675a46a24d

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

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