Run xhpast in parallel

Summary: I wonder if this is not by purpose?

Test Plan: Modified two files, ran `arc lint`.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2629

src/lint/linter/xhpast/ArcanistXHPASTLinter.php

@@ -137,7 +137,7 @@ final class ArcanistXHPASTLinter extends ArcanistLinter {
     foreach ($paths as $path) {
       $futures[$path] = xhpast_get_parser_future($this->getData($path));
     }
-    foreach ($futures as $path => $future) {
+    foreach (Futures($futures)->limit(8) as $path => $future) {
       $this->willLintPath($path);
       try {
         $this->trees[$path] = XHPASTTree::newFromDataAndResolvedExecFuture(