Strip tips out of commit messages from arc backout

Summary:
Fixes T10707. Currently, `arc backout` creates a commit message which includes questionably-helpful "tips" in the message itself.

Strip these out.

Test Plan:
Used `arc backout` to revert any commit, then `git show` to see the generated message.

  - Before patch: included tips.
  - After patch: no tips.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10707

Differential Revision: https://secure.phabricator.com/D15573

src/workflow/ArcanistBackoutWorkflow.php

@@ -123,6 +123,9 @@ EOTEXT
     $template = $this->newInteractiveEditor($template)
       ->setName('new-commit')
       ->editInteractively();
+
+    $template = ArcanistCommentRemover::removeComments($template);
+
     return $template;
   }