$ git clone -n https://src.fedoraproject.org/rpms/antlr3.git /var/lib/mock/f31-build-102144-33490/root/chroot_tmpdir/scmroot/antlr3
Cloning into '/var/lib/mock/f31-build-102144-33490/root/chroot_tmpdir/scmroot/antlr3'...
$ git reset --hard d6b59940717e6a76bc3e48f610d29c604a2267a5
HEAD is now at d6b5994 BR ant to fix FTBFS.  Thanks to Fabio Valentini for the hint.