Add 'git p' as the equivalent of 'git co', but for pulling instead of checkout.

Summary:
Without this, it's really annoying -- and sometimes bug-inducing -- to
do a 'git pull' in master, and have it not update substate.  I used to
do 'git pull && git co <current-branch>' which is ridiculous.
Hopefully I'll be able to get into the habit of doing 'git p' instead
of 'git pull', just like I do 'git co' instead of 'git checkout'.

Test Plan: Ran 'git p' and saw it do some pulling.

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D2619