In the "create new repos" loop, we need an absolute value for REPO_BASE, in
order to be able to chdir back and forth. But (taking the "normal user with
no privileges" assumption too far!) we assumed REPO_BASE would be within
$HOME, and relative to it. So it fails when someone wants the repo_base
elsewhere.
Now we don't prefix $HOME if REPO_BASE is already absolute (begins with a "/")
bug reported by evocallaghan
