Fix cpplint message regex

Authored by jra3.

Description

Fix cpplint message regex

Summary:
ref T8404

The issue is caused, not by the non-zero return code, but by the fact that
$messages is coming back empty due to the incorrect regex. tyhoff pointed out
that the regex matched correctly when we used STDIN, but now it is failing.

https://secure.phabricator.com/diffusion/ARC/browse/master/src/lint/linter/ArcanistExternalLinter.php;8c589f1f759f0913135b8cc6959a6c1589e14ae4$357

Test Plan:
arc lint cpp file containing lint error. run cpplint on the
file directly to confirm that there are errors and that the return code
is non-zero

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin

Maniphest Tasks: T8404

Differential Revision: https://secure.phabricator.com/D14960