This avoids the problem in which appending to a slice returned by
Split can affect subsequent slices.

Fixes #21149.

Change-Id: Ie3df2b9ceeb9605d4625f47d49073c5f348cf0a1
Reviewed-on: https://go-review.googlesource.com/74510Reviewed-by: Jelte Fennema <github-tech@jeltef.nl>
Reviewed-by: Robert Griesemer <gri@golang.org>