Add a utility function to do a clone-or-update.

I wsa doing this complicated
    git clone || (cd ... && git pull)
all over the place, and I realized when installing the new toby that I
really needed to do
    git --recursive clone || (cd ... && git pull)

I was like, "if there was a utility function to do this then I'd only
need to change one place!"  Well, now there is, and I changed that one
place.

Auditors: chris

Test Plan:
Fingers crossed.  I'll try running setup.sh on the new toby and see
how it goes.