Add 'git p' and 'git co' aliases to the completion file.

Test Plan:
Pasted these two lines into bash manually:
   _git_p() { _git_pull; }
   _git_co() { _git_checkout; }
and then did
   git p <tab>
and saw it complete to 'origin'.  And did
   git co <tab>
and saw it suggest things like 'master', 'HEAD', etc.

Reviewers: desmond

Reviewed By: desmond

Subscribers: jlfwong

Differential Revision: http://phabricator.khanacademy.org/D10104