$ git clone -n https://src.fedoraproject.org/rpms/R-FMStable.git /var/lib/mock/f42-build-side-1-3500322-45970/root/chroot_tmpdir/scmroot/R-FMStable
Cloning into '/var/lib/mock/f42-build-side-1-3500322-45970/root/chroot_tmpdir/scmroot/R-FMStable'...
$ git fetch origin 94860abd8bdc983f83d08050a6929791f16b9754:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/R-FMStable
 * [new ref]         94860abd8bdc983f83d08050a6929791f16b9754 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at 94860ab R-maint-sig mass rebuild