$ git clone -n https://src.fedoraproject.org/rpms/R-png.git /var/lib/mock/f42-build-side-1-3501289-45974/root/chroot_tmpdir/scmroot/R-png
Cloning into '/var/lib/mock/f42-build-side-1-3501289-45974/root/chroot_tmpdir/scmroot/R-png'...
$ git fetch origin f79f46e103bfc821f35ac01e1af890ad421b028b:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/R-png
 * [new ref]         f79f46e103bfc821f35ac01e1af890ad421b028b -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at f79f46e R-maint-sig mass rebuild