The ancestor doesn't always match with the first.

Change-Id: I0edcbffab7e19ba1731e849021ffbb7428ec48d7
Reviewed-on: https://go-review.googlesource.com/c/161857
Run-TryBot: Kunpei Sakai <namusyaka@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>