Fix update-submodule to work in the world of no-commit-to-master.

Summary:
This used to automatically change to the master branch before doing
its magic.  That's not a good idea anymore, since you can't commit to
master.  Instead, we now require you to change to the right branch
before running 'git update-submodule'.  (It will update the submodule
in your current branch.)

Test Plan: Fingers crossed.

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D13102