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