Strip comments during `arc diff --verbatim` workflow before submitting to Phabricator

Summary: Fixes T4649. The issue in that task is caused because we're submitting a block of text including comments. We've probably been doing this for a long time, but maybe were more liberal in parsing before. Instead, strip them.

Test Plan: Ran `arc diff --verbatim` and got "Dxxx" prefilled correctly.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4649

Differential Revision: https://secure.phabricator.com/D8658