git-codereview: reject commits with multiple Change-Id lines

This should help prevent the situation in CL 8653.

Change-Id: I03e4868656a86e69e11f3e0545feb1681fb0f628
Reviewed-on: https://go-review.googlesource.com/8671
Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
3 files changed