$ git clone -n https://src.fedoraproject.org/rpms/R-base64enc.git /var/lib/mock/f42-build-side-1-3500311-45970/root/chroot_tmpdir/scmroot/R-base64enc
Cloning into '/var/lib/mock/f42-build-side-1-3500311-45970/root/chroot_tmpdir/scmroot/R-base64enc'...
$ git fetch origin e6902f51efe4a3d8104654b7952fa17cdc4ffe40:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/R-base64enc
 * [new ref]         e6902f51efe4a3d8104654b7952fa17cdc4ffe40 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at e6902f5 R-maint-sig mass rebuild