Add an 'update-submodule' alias to make it easy to update submodules.

Summary:
The control flow is now:
   1) go to submodule (or check out the submodule-repo somewhere else)
   2) make edits in submodule
   3) review/etc
   4) commit submodule changes
   5) push submodule changes
   6) go to webapp-root
   7) run 'git update-submodule path/to/submodule'

Step (7) used to be a lot of steps, but now it's just 1!

Test Plan:
I've been using this alias myself, successfully, for several months.
Now I'm just sharing it with the world.

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D5572