fix differential + diffusion for showing generated files

Summary: in the re-factor quest this got broken. Turns out NewLines (aka $this->new in the parser) gets updated right up until the very end for showing text changes so instead pass over the total line count to the renderer to get this codepath right.

Test Plan: showed content of some generated files in diffusion and all was well

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2204

Differential Revision: https://secure.phabricator.com/D4237