(T1889) arc cover supports Mercurial too

Summary: improving documentation: add cover supports Mercurial

Test Plan: ./bin/arc help cover

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1889

Differential Revision: https://secure.phabricator.com/D3683

src/workflow/ArcanistCoverWorkflow.php

@@ -32,7 +32,7 @@ public function getCommandSynopses() {
 
   public function getCommandHelp() {
     return phutil_console_format(<<<EOTEXT
-          Supports: svn, git
+          Supports: svn, git, hg
           Cover your... professional reputation. Show blame for the lines you
           changed in your working copy (svn) or since some commit (hg, git).
           This will take a minute because blame takes a minute, especially under