Make sure we do a submodule update immediately after a merge.

Summary:
As mentioned in http://codingkilledthecat.wordpress.com/2012/04/28/why-your-company-shouldnt-use-git-submodules/:

| When Git drops into conflict resolution mode, it still doesn’t update the submodule pointers – which means that when you commit the merge after resolving conflicts, you run into the same problem as in the previous section: if you forgot to run git submodule update, you’ve just reverted any submodule commits the branch you merged in might have made.

With this alias, you can't 'forget' to run git submodule update,
because we do it for you!

Test Plan: None

Reviewers: alpert

Reviewed By: alpert

Subscribers: leah

Differential Revision: http://phabricator.khanacademy.org/D11452