Display number of lint messages in Diffusion dir

Test Plan:
Went to https://secure.phabricator.com/diffusion/ARC/browse/.
Saw number of lint messages per dir, not terribly slow.
Went to https://secure.phabricator.com/diffusion/ARC/browse/master/src/difference/.
Saw number of lint messages per file.
Disabled lint, didn't see it.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3921