Without passing --allow-unrelated-histories the command sequence
fails as intended since commit e379fdf34f ("merge: refuse to create
too cool a merge by default"). To setup a subtree merging unrelated
histories is normal, so add the option to the howto document.
Signed-off-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
