$ git clone -n https://src.fedoraproject.org/rpms/R-tufte.git /var/lib/mock/f42-build-side-1-3502092-45977/root/chroot_tmpdir/scmroot/R-tufte
Cloning into '/var/lib/mock/f42-build-side-1-3502092-45977/root/chroot_tmpdir/scmroot/R-tufte'...
$ git fetch origin 8a9252b6bc2281a85ac41d14082fd8beee953c4e:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/R-tufte
 * [new ref]         8a9252b6bc2281a85ac41d14082fd8beee953c4e -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at 8a9252b R-maint-sig mass rebuild