Throw properly if there are uncommitted changes under Git

Summary: This was accidentally disabled with some Mercurial changes that allowed dirty working copies.

Test Plan: Ran `arc diff` with staged changes.

Reviewers: nh

Reviewed By: nh

CC: aran

Differential Revision: https://secure.phabricator.com/D3165

src/workflow/ArcanistDiffWorkflow.php

@@ -578,6 +578,8 @@ EOTEXT
 
           $repository_api->setIncludeDirectoryStateInDiffs(true);
           $this->haveUncommittedChanges = true;
+        } else {
+          throw $ex;
         }
       }
     }