$ git clone -n https://github.com/xcp-ng-rpms/xen /var/lib/mock/v8.3-incoming-9508-28242/root/chroot_tmpdir/scmroot/xen
Cloning into '/var/lib/mock/v8.3-incoming-9508-28242/root/chroot_tmpdir/scmroot/xen'...
$ git fetch origin 88dd81ceae771c238ef11ee32ce3c07f0b53a094:KOJI_FETCH_HEAD
From https://github.com/xcp-ng-rpms/xen
 * [new ref]         88dd81ceae771c238ef11ee32ce3c07f0b53a094 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at 88dd81c Merge pull request #73 from xcp-ng-rpms/xsa-475