Update Phabricator for split syntax highlighting API

Summary: I'll clean some of this stuff up in a followup too, but update the
callers to use the new explicit filename-based API.
Test Plan: Looked at paste, Diffusion and Differential.
Reviewed By: tuomaspelkonen
Reviewers: tuomaspelkonen, codeblock, jungejason, aran
CC: aran, tuomaspelkonen
Differential Revision: 600