The -diff flag causes tidy not to modify the files but instead print the
necessary changes as a unified diff. It exits with a non-zero code
if updates are needed.
Fixes: #27005
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest,gotip-windows-amd64-longtest
Change-Id: Ie239367f2fc73ecb55ec2ce76442293635c1b47d
Reviewed-on: https://go-review.googlesource.com/c/go/+/585401 Reviewed-by: Michael Matloob <matloob@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>