The following happen trying to perform a clean checkout:
git clone http://git-wip-us.apache.org/repos/asf/incubator-s4.git
Cloning into 'incubator-s4'...
remote: Counting objects: 9156, done.
remote: Compressing objects: 100% (3029/3029), done.
remote: Total 9156 (delta 4696), reused 8496 (delta 4425)
Receiving objects: 100% (9156/9156), 18.12 MiB | 635 KiB/s, done.
Resolving deltas: 100% (4696/4696), done.
warning: remote HEAD refers to nonexistent ref, unable to checkout.
Following the suggestion of Harsh J, i tried to go into the created incubator-s4 directory (empty) doing:
git checkout trunk
error: pathspec 'trunk' did not match any file(s) known to git.