Make sure git_clone() is run as root

Only root (and the original user who ssh'd in) can access the SSH agent
socket.
git_clone(branch=...)
A warning hidden in some README is useless

An error message that tells you what to do is useful.

(It can also be annoying -- "computer, if you know what command I should
run, why don't you just run it yourself!" -- but less annoying than
guessing wrong.)