git-codereview: skip git tests when there's no git

To fix linux builders for golang/go#11811.

Change-Id: I8058e09bc39c374ab9af469b2a6c732be14453f2
Reviewed-on: https://go-review.googlesource.com/13692
Reviewed-by: Russ Cox <rsc@golang.org>
1 file changed