[arcanist] respect severity disabled in PEP8 linter

Summary:
This allows for disabling certain PEP8 linter errors by calling
setCustomSeverityMap on an ArcanistPEP8Linter. However, any custom severities
besides disabled will be ignored.

Test Plan: arc lint

Reviewers: epriestley, andrewjcg

Reviewed By: epriestley

CC: jungejason, aran, epriestley

Differential Revision: https://secure.phabricator.com/D1839

src/lint/linter/pep8/ArcanistPEP8Linter.php

@@ -122,6 +122,9 @@ final class ArcanistPEP8Linter extends ArcanistLinter {
       $message->setCode($matches[4]);
       $message->setName('PEP8 '.$matches[4]);
       $message->setDescription($matches[5]);
+      if (!$this->isMessageEnabled($matches[4])) {
+        continue;
+      }
       if ($matches[4][0] == 'E') {
         $message->setSeverity(ArcanistLintSeverity::SEVERITY_ERROR);
       } else {