Fix arc lint amend for git to not add untracked files

Summary:
Arc lint for git is currently adding all untracked files when it
amends the commit with lint fixes. This changes the git add -A to be
git add -u. This only adds files that were already tracked. -A was adding
untracked files as well which was not the desired behavior here.

Test Plan:
Create an untracked file.
Commit a lint failure another file.

arc diff and choose to amend the lint patches.

Verify that the untracked file was not added but the tracked file was amended.

Reviewers: epriestley, wez, chad

Reviewed By: chad

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5731

src/workflow/ArcanistLintWorkflow.php

@@ -481,7 +481,7 @@ EOTEXT
       if ($amend) {
         if ($repository_api instanceof ArcanistGitAPI) {
           // Add the changes to the index before amending
-          $repository_api->execxLocal('add -A');
+          $repository_api->execxLocal('add -u');
         }
 
         $repository_api->amendCommit();