$ git clone -n https://src.fedoraproject.org/rpms/R-MatrixGenerics.git /var/lib/mock/f45-build-5898-345/root/chroot_tmpdir/scmroot/R-MatrixGenerics
Cloning into '/var/lib/mock/f45-build-5898-345/root/chroot_tmpdir/scmroot/R-MatrixGenerics'...
$ git fetch origin f4d6e44f085bea23b0753d461b77fd0b63e9585e:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/R-MatrixGenerics
 * [new ref]         f4d6e44f085bea23b0753d461b77fd0b63e9585e -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at f4d6e44 Rebuilt for https://fedoraproject.org/wiki/Fedora_45_Mass_Rebuild