When a file doesn't begin "<?php", raise one warning, not one for each line

Summary: Linter flipped out on D1817; reign it in.

Test Plan: Ran "arc lint" on D1817, got one message.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1818

src/lint/linter/xhpast/ArcanistXHPASTLinter.php

@@ -686,6 +686,7 @@ final class ArcanistXHPASTLinter extends ArcanistLinter {
             'PHP files should start with "<?php", which may be preceded by '.
             'a "#!" line for scripts.');
         }
+        break;
       }
     }
     foreach ($tokens as $token) {

src/lint/linter/xhpast/__tests__/data/php-tags-bad.lint-test

@@ -4,11 +4,9 @@ garbage garbage
 ?>
 ~~~~~~~~~~
 error:1:1
-error:2:1
 error:4:1
 ~~~~~~~~~~
 garbage garbage
-<?php
-
+<?
 
 ?>