$ git clone -n https://src.fedoraproject.org/rpms/R-broom.git /var/lib/mock/f40-build-1930457-25111/root/chroot_tmpdir/scmroot/R-broom
Cloning into '/var/lib/mock/f40-build-1930457-25111/root/chroot_tmpdir/scmroot/R-broom'...
$ git fetch origin d54c303576718a2d950a23176607c8f5ab9ff38e:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/R-broom
 * [new ref]         d54c303576718a2d950a23176607c8f5ab9ff38e -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at d54c303 R-maint-sig mass rebuild