Update command line tools link and check for newer git version

Test Plan:
I did a "brew uninstall git", then ran mac-setup.sh and opened a new shell window. Git successfully updated to 1.8.2.2.
(see discussion over at http://phabricator.khanacademy.org/D2252)

Reviewers: csilvers

Reviewed By: csilvers

Differential Revision: http://phabricator.khanacademy.org/D2268