Fix an issue with "arc diff --update" in Git on immutable workflows

Summary:
This should be a raw text block, not a parsed message object.

I broke this in rARC1f13e022cdc9bf4859274a83784bd615caf62ef9 when I improved Mercurial support.

See: https://github.com/facebook/arcanist/issues/23

Test Plan: (Will update this diff...)

Reviewers: vrana, jungejason, btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D2136

src/workflow/diff/ArcanistDiffWorkflow.php

@@ -1777,7 +1777,7 @@ EOTEXT
       }
 
       // Otherwise, this looks new, so it's a usable commit message.
-      $usable[] = $message;
+      $usable[] = $text;
     }
 
     if (!$usable) {