$ git clone -n https://github.com/xcp-ng-rpms/xen.git /var/lib/mock/v8.3-u-prebuild-9262-28049/root/chroot_tmpdir/scmroot/xen
Cloning into '/var/lib/mock/v8.3-u-prebuild-9262-28049/root/chroot_tmpdir/scmroot/xen'...
$ git fetch origin 0efef6134b8bddea4cc5ad1dafd21f1d4e9bbaa6:KOJI_FETCH_HEAD
From https://github.com/xcp-ng-rpms/xen
 * [new ref]         0efef6134b8bddea4cc5ad1dafd21f1d4e9bbaa6 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at 0efef61 bump release for test build