$ git clone -n https://github.com/xcp-ng-rpms/xs-opam-repo.git /var/lib/mock/v8.3-u-psafont2-9641-28361/root/chroot_tmpdir/scmroot/xs-opam-repo
Cloning into '/var/lib/mock/v8.3-u-psafont2-9641-28361/root/chroot_tmpdir/scmroot/xs-opam-repo'...
$ git fetch origin 3555dcebfa3656bdbcf393e08b508a944d61293c:KOJI_FETCH_HEAD
From https://github.com/xcp-ng-rpms/xs-opam-repo
 * [new ref]         3555dcebfa3656bdbcf393e08b508a944d61293c -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at 3555dce bump release for test build