$ git clone -n https://github.com/xcp-ng-rpms/xen /var/lib/mock/v8.3-incoming-9137-27953/root/chroot_tmpdir/scmroot/xen
Cloning into '/var/lib/mock/v8.3-incoming-9137-27953/root/chroot_tmpdir/scmroot/xen'...
$ git fetch origin 54da6b8c6a5bfdbab96660bac5b608477b6df646:KOJI_FETCH_HEAD
From https://github.com/xcp-ng-rpms/xen
 * [new ref]         54da6b8c6a5bfdbab96660bac5b608477b6df646 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at 54da6b8 hvmloader: Add SMBIOS 2.6 patch and legacy option