Flush output before printing lint and unit results in diff

Summary:
Under HPHP, output buffer size is ignored.
Flush any buffered output before printing anything from arc diff to reduce user's confusion.

Test Plan:

$ arc diff --preview --background 1
$ arc diff --preview --background 0

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3543