Show cross-references in diffusion

Summary:
We do this in differential. To do this in diffusion, we need to know the
arcanist project (which I do by loading all possible projects for the
repository) and the language.

Test Plan:
load a php file in diffusion to see crossreferences; load a text file and
check darkconsole that it didn't try to crossreference.

Reviewers: epriestley, vrana, jungejason

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3020