Set branch.autosetuprebase = always.  We're a rebase shop!

Test Plan:
Set this in my local .gitconfig and ran
   git co -b test2
and it said
   Branch test2 set up to track local branch master by rebasing.
which seems right to me.

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D2168