Log two times per linter instead of N

Summary: This number seems more interesting and it includes time for resolving futures which is the main part of some linters.

Test Plan:
  $ arc --trace lint

Reviewers: fdeliege, epriestley

Reviewed By: epriestley

CC: aran, epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D5075