$ git clone -n https://src.fedoraproject.org/rpms/postfix.git /var/lib/mock/f39-build-2184718-51281/root/chroot_tmpdir/scmroot/postfix
Cloning into '/var/lib/mock/f39-build-2184718-51281/root/chroot_tmpdir/scmroot/postfix'...
$ git fetch origin 1e3971d1c49992a49aa15b08ee7fd6564a4171a0:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/postfix
 * [new ref]         1e3971d1c49992a49aa15b08ee7fd6564a4171a0 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at 1e3971d New version