$ git clone -n https://src.fedoraproject.org/rpms/R-packrat.git /var/lib/mock/f40-build-1944523-25177/root/chroot_tmpdir/scmroot/R-packrat
Cloning into '/var/lib/mock/f40-build-1944523-25177/root/chroot_tmpdir/scmroot/R-packrat'...
$ git fetch origin fe96bccb4488b6583086aa8861bd29b87e717ab0:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/R-packrat
 * [new ref]         fe96bccb4488b6583086aa8861bd29b87e717ab0 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at fe96bcc R-maint-sig mass rebuild