$ git clone -n https://github.com/xcp-ng-rpms/xs-opam-repo.git /var/lib/mock/v8.3-u-psafont2-9745-28457/root/chroot_tmpdir/scmroot/xs-opam-repo
Cloning into '/var/lib/mock/v8.3-u-psafont2-9745-28457/root/chroot_tmpdir/scmroot/xs-opam-repo'...
$ git fetch origin b51822ea0e60d3870c93e46399eb5be27fdd0218:KOJI_FETCH_HEAD
From https://github.com/xcp-ng-rpms/xs-opam-repo
 * [new ref]         b51822ea0e60d3870c93e46399eb5be27fdd0218 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at b51822e bump release for test build