$ git clone -n https://src.fedoraproject.org/rpms/firefox.git /var/lib/mock/f40-build-1812237-24896/root/chroot_tmpdir/scmroot/firefox
Cloning into '/var/lib/mock/f40-build-1812237-24896/root/chroot_tmpdir/scmroot/firefox'...
$ git fetch origin 70d0fad2399e61d62cd45c13f7f10fa513fe321c:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/firefox
 * [new ref]         70d0fad2399e61d62cd45c13f7f10fa513fe321c -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at 70d0fad Updated cbindgen