Use --allow-empty when git amending to HEAD

Summary: ... in case the head commit is empty. Empty commits are useful for
injecting an Arcanist commit message in a branch then sending the whole thing
off for review. As far as I know there is no situation in which an empty commit
would exist unintentionally and using ##--allow-empty## would suppress an error.

Test Plan: works fine for a branch I just sent for review

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: 1241

src/repository/api/git/ArcanistGitAPI.php

@@ -321,7 +321,7 @@ class ArcanistGitAPI extends ArcanistRepositoryAPI {
 
   public function amendGitHeadCommit($message) {
     execx(
-      '(cd %s; git commit --amend --message %s)',
+      '(cd %s; git commit --amend --allow-empty --message %s)',
       $this->getPath(),
       $message);
   }