Speed up DiffusionBrowseFileController by removing call to array_merge

Summary: Some profiling using XHProf in the Dark Console showed me that Diffusion was wasting a ton of time on array_merge. This change sped up the loading of a large file in Diffusion from 16.8 seconds to 2.4 seconds.

Test Plan: Load files in Diffusion. They all look good. Also, use a PHP shell to try to manually verify that I still kinda remember some PHP and, yes, this is functionally equivalent to what was there before.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9032