$ git clone -n https://src.fedoraproject.org/rpms/R-rex.git /var/lib/mock/f42-build-side-110318-58926213-6569309/root/chroot_tmpdir/scmroot/R-rex
Cloning into '/var/lib/mock/f42-build-side-110318-58926213-6569309/root/chroot_tmpdir/scmroot/R-rex'...
$ git fetch origin 04ff1fef9f545f7b0d597dbeff95ffd26d8a6921:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/R-rex
 * [new ref]         04ff1fef9f545f7b0d597dbeff95ffd26d8a6921 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at 04ff1fe R-maint-sig mass rebuild