Since a root certificate is self-signed, it's a valid child of itself.
If a root certificate appeared both in the pool of intermediates and
roots the verification code could find a chain which included it twice:
first as an intermediate and then as a root. (Existing checks prevented
the code from looping any more.)

This change stops the exact same certificate from appearing twice in a
chain. This simplifies the results in the face of the common
configuration error of a TLS server returning a root certificate.

(This should also stop two different versions of the “same” root
appearing in a chain because the self-signature on one will not validate
for the other.)

Fixes #16800.

Change-Id: I004853baa0eea27b44d47b9b34f96113a92ebac8
Reviewed-on: https://go-review.googlesource.com/32121
Run-TryBot: Adam Langley <agl@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>