Examine only HEAD in amend workflow

Summary: We don't support `git rebase -i` or other magic in `arc amend` so we can examine just HEAD which we always amend.

Test Plan: `arc amend` on a branch with two commits.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2643

src/workflow/ArcanistAmendWorkflow.php

@@ -99,6 +99,8 @@ EOTEXT
       $revision_id = $this->normalizeRevisionID($this->getArgument('revision'));
     }
 
+    $repository_api->setRelativeCommit('HEAD^');
+
     $in_working_copy = $repository_api->loadWorkingCopyDifferentialRevisions(
       $this->getConduit(),
       array(