Move complicated aliases to their own files

Summary:
Git pretty much requires you to understand what it's doing at a low level to
understand how to use it. These aliases weren't very readable inside of
.gitconfig all on one line. So this change moves them to ./bin and also makes
sure that directory is added to people's paths.

Hopefully that makes these commands less of a black box.

Test Plan:
 * Open a new terminal window.
 * `echo $PATH` to make sure that ./bin was there.
 * Run `git deploy-branch` and `git review-branch` without error.

Reviewers: csilvers

Reviewed By: csilvers

Subscribers: alpert

Differential Revision: http://phabricator.khanacademy.org/D11158