$ git clone -n https://github.com/xcp-ng-rpms/xen.git /var/lib/mock/v8.3-incoming-8879-27828/root/chroot_tmpdir/scmroot/xen
Cloning into '/var/lib/mock/v8.3-incoming-8879-27828/root/chroot_tmpdir/scmroot/xen'...
$ git fetch origin b6c6e5c41144f292babb423adfc44efe09258da5:KOJI_FETCH_HEAD
From https://github.com/xcp-ng-rpms/xen
 * [new ref]         b6c6e5c41144f292babb423adfc44efe09258da5 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at b6c6e5c x86/hvmloader: select xen platform pci MMIO BAR UC or WB MTRR cache attribute