Turn off write guard when saving differential comment cache

Summary:
It is possible to view a comment that has no cache; when viewing such a comment
the request doesn't have a csrf token and there is no need for one, so we turn
off the write guard.

Test Plan:
loaded an old diff that had no cache, and the page loaded instead of throwing
an AphrontCSRFException.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: 858

src/applications/differential/view/revisioncomment/DifferentialRevisionCommentView.php

@@ -195,7 +195,9 @@ final class DifferentialRevisionCommentView extends AphrontView {
                 $inline_content);
               if ($inline->getID()) {
                 $inline->setCache($inline_content);
+                $unguarded = AphrontWriteGuard::beginScopedUnguardedWrites();
                 $inline->save();
+                unset($unguarded);
               }
             }
           }